UNIVERSITY  OF  CALIFORNIA 
LOS  ANGELES 


GIFT  OF 

James  W.  Moncrieff 


t 


V» 


3  O  <b 


DRYING  CLAY  WARES 

BY  ELLIS  LOVEJOY,  E.M. 


Member  and  Ex-President 
American   Ceramic   Society. 

Member 

American  Institute  of  Mining  Engineers, 
National  Brick  Manufacturers'  Association. 


T.  A.  RANDALL  &  CO. 

Publishers, 
Indianapolis,  Ind. 


Copyrighted  1916 

by 
A.  RANDALL  &  CO. 


TP 


PREFACE 

The  following  articles  which  appeared  serially  in  The 
Clay-Worker  were  inspired  by  the  fact  that  there  is  no  pub- 
lished literature  on  the  subject  of  drying  clay  wares. 

The  various  treatises  on  heating,  ventilation  and  drying 
include  only  in  a  small  measure  data  applicable  to  our  clay 
drying  problems.  In  our  engineering  work  we  have  collected, 
modified  and  applied  this  data  to  our  special  needs,  and  the 
articles  include  such  engineering  data  as  we  have  found 
useful. 

We  have  tried  to  make  the  articles  not  merely  descriptive, 
nor  yet  too  technical,  and  hope  they  will  prove  of  some 
assistance  to  both  the  practical  and  technical  clayworker. 

Credit  is  due  Mr.  T.  W.  Garve,  M.  E.,  for  all  the  pen  and 
ink  drawings  and  for  the  chapter  on  German  clayworking 
plants.  We  also  wish  to  acknowledge  the  assistance  of  Prof. 
C.  B.  Harrop,  E.  M.,  in  numerous  discussions  of  the  problems 
involved  and  in  checking  over  and  correcting  the  mathe- 
matical work. 

Credit  has  been  given  in  the  text  to  all  authorities  as  far 
as  possible,  though  in  many  instances  the  information  is  from 
our  engineering  data  and  so  interwoven  with  it  that  it  is  im- 
possible to  trace  it  to  any  single  authority. 

ELLIS  LOVEJOY,  E.  M. 

Columbus,  Ohio,  January  22,  1916. 


TABLE  OF  CONTENTS 

CHAPTER  I. 

Page 
Classification  of  Clays  ...................................     7 

Water  in  Clay  .............................  g 

How  Drying  Proceeds  ................................     9 

CHAPTER  II. 
The  Work  to  Be  Done  ...................  12 

CHAPTER  III. 
The  Relation  of  Air  to  Drying  ......................  16 

Effect  of  Lamination  on  Drying  ................  19 

'' 


_  ........  ..............    20 

Preheating  Clay    ...........    ..............  21 


CHAPTER  IV. 
CHAPTER  V. 


Shrinkage    .  .  . 

....................  .     Zo 


Air  Drying  ............    ...............  33 

Open  Yard  Drying  of  Soft  Mud  Brick  ........ 

Stiff  Mud  Brick  in  Open  Yard  .........  '35 

Rack  and  Pallet  Yard  ................. 

Air  Drying  Stiff  Mud  Brick.. 

Drain  Tile  Sheds  .....................  '""  '    43 

CHAPTER  VI. 
Drying  Above  Continuous  Kilns  ........ 


Advantages 

Disadvantages    .  '  •  '  ' 

'  '  •  ......................    61 

CHAPTER  VII. 
Artificial  Dryers 

General  Principles   ...... 

Floor  Dryers—  Hot  Floor  ]  '  . 


CHAPTER  VIII. 
Sewer  Pipe  Floors 62 

CHAPTER  IX. 

Periodical  Dryers 78 

Steam  Pipe  Dryers _ _ 78 

The  Boss  Dryer 103 

The  Pipe  Rack  Dryer 105 

Tender  Clay  Dryer 107 

CHAPTER  X. 
Pottery  Dryer 110 

CHAPTER  XI. 

Terra  Cotta  and  Other  Special  Dryers 115 

The  Scott  System 118 

Car  Tunnel  Kilns _ 120 

CHAPTER  XII. 

Conservation  of  Heat  in  German  Factories 124 

Moeller  and  Pfieffer  Dryer 127 

CHAPTER  XIII. 
Progressive  Dryers 130 

CHAPTER  XIV. 
Radiated   Heat  Dryers 133 

CHAPTER  XV. 
Steam  Progressive  Dryers ,  141 

CHAPTER  XVI. 

Waste  Heat  Progressive  Dryer 145 

Air    Volume    of    Heat    Requirement    for   Waste    Heat 

Dryers 154 

Another  Method  of  Determining  Air  Volume 159 


DRYING    CLAY    WARES 


DRYING  CLAY  WARES. 

By  Ellis  Lovejoy. 


CHAPTER  I. 
Classification  of  Clays. 

DRYING   CLAY   WARES    is   one    of   the   most   complex 
problems  with  which  the  clayworker  has  to  deal,  and 
one  that  is  least  considered  and  least  understood.    We 
are  lacking  in  a  proper  classification  of  clays  in  their  relation 
to  drying  and  perhaps  no  accurate  classification  is  possible. 
Clays  may  be  roughly  classified  as-  follows: 

(1)  Clays  that  may  be  dried  in  a  few  hours,  starting  with 
an  initial  high  temperature  and  rapid  drying  condition. 

(2)  Clays  that  may  be  dried  in  twenty-four  hours  or  less, 
starting  at  an  initial  low  temperature  in  saturated  atmosphere. 

(3)  Clays  that  can   be   dried  in   twenty-four  to  seventy- 
two  hours,  under  conditions  of  No.  2. 

(4)  Clays    that    require    over    seventy-two    hours    under 
conditions  of  No.  2. 

(5)  Clays  that  must  be  slowly  heated  up  in  a  saturated 
stagnant  atmosphere  before  advancing  into  a  moving  drying 
atmosphere. 

(6)  Clays  that  cannot  safely  be  subjected  to  conditions 
under  No.  2  and  No.  5,  but  must  be  dried  slowly,  starting  un- 
der drying  conditions. 

(7)  Clays  that  cannot  be  dried  under  any  condition. 
There  are  many  clays  and  shales  which  will  rate  as  No.  1 ; 

clays  that  can  be  dried  in  a  pipe  rack  dryer;  that  will  stand 
exposure  to  wind  and  sun;   that  can  be  placed  in  a  periodic 


DRYING    CLAY    WARES 


dryer  in  which  a  maximum  temperature  is  maintained  from 
end  to  end  of  dryer. 

No.  2  contains  a  greater  number  of  clays  and  the  pro- 
gressive dryers  in  which  No.  2  conditions  prevail,  are  the  most 
numerous.  No.  2  class  shades  into  No.  3  and  No.  4. 

There  are  many  tender  drying  clays  that  can  be 
dried  safely  by  first  heating  them  in  a  saturated  atmos- 
phere, then  moving  them  into  conditions  No.  2  and  No.  4. 
No.  5  is  considered  the  .method  par  excellence  for  very  tender 
clays,  but  we  have  found  some  clays  that  can  be  safely  dried 
under  somewhat  trying  conditions,  provided  they  are  not 
first  subjected  to  a  moist  atmosphere — the  humidity  treat- 
ment which  prevails  in  No.  2,  3.  4,  and  especially  in  No.  5. 

These  classes  include  many  clays,  but  there  are  many 
others,  perhaps  as  many,  that  cannot  be  dried  under  any 
conditions  in  their  raw  state,  but  must  first  be  subjected  to 
some  treatment  which  changes  the  physical  conditions. 

A  weak  point  in  any  classification  is  that  a  clay  which 
may  be  difficult  to  dry  in  one  kind  of  ware  becqmes  easy 
to  dry  in  some  other  ware.  We  have  frequently  found  clays 
which  in  bricks  required  thirty-six  to  forty-eight  hours  to 
dry  safely,  yet  in  drain  tile  would  dry  without  a  flaw  in 
twenty-four,  or  even  twelve  hours.  In  safe  drying,  drain  tile 
easily  leads  the  list,  then  comes  sewer  pipe  in  small  sizes, 
simple  and  small  hollow  ware  especially  in  clays  that  lamin- 
ate, bricks,  perforated  bricks,  complex  hollow  ware,  large 
sewer  pipe,  roofing  tile,  terra  cotta,  etc. 

Water   in   Clay. 
*Water  in  clay  exists  in  three  conditions: 

(1)  Moisture,  fa)  free  water,  fb)  water  of  saturation. 

(2)  Hygroscopic  water. 

(3)  Chemically  combined  water. 
We  might  also  add  colloidal  water. 

Free  water  is   the  water  which   fills   the   pore    spaces   in 

the  clay  mass,  and  which   serves   as  a  lubricant,  permitting 

ie  grains  of  clay  to  slip  one  on  another,  and  gives  the  mass 

the   mobility    necessary   for    casting    or   moulding   processes 

the   water  with  which   we   are   chiefly  concerned   in 

*0ne  authority  classifies  the  water  in  clav 

sssSr^ 

y,  hygroscopic  water,  and  combined  or  ch4mi        wate 


DRYING    CLAY    WARES 


drying.  When  it  is  driven  off  the  clay  mass  is  rigid;  it  will 
no  longer  flow  under  pressure,  but  it  still  looks  wet. 

Water  of  saturation  is  that  water  clinging  to  the  surfaces 
of  the  grains  of  clay  after  the  pores  have  been  emptied  of 
their  free  waters.  This  water  can  only  be  removed  by  con- 
tact with  some  other  substance  having  greater  affinity  for  the 
water  than  that  of  the  clay,  or  by  vaporization  from  the  sur- 
faces of  the  clay  grains. 

We  say  clay  wares  are  bone  dry  when  they  come  from 
the  dryer,  showing  no  perceptible  wetness.  Yet  we  know 
that  in  the  water-smoking,  there  is  driven  off  from  the  dry 
wares  a  considerable  volume  of  water  vapor.  This  is  hy- 
groscopic water  and  also  colloidal  water.  We  have  no  hint 
of  its  presence  either  in  the  appearance  or  feel  of  the  clay. 
We  often  use  the  expression  "dry  as  dust"  to  express  dry- 
ness,  but  dust  contains  hygroscopic  water. 

Combined  water  is  a  chemical  constituent  of  clay  and 
only  appears  when  we  disintegrate  the  mineral  kaolinite  by 
heat. 

To  recapitulate:  Free  water  is  sensible  moisture  entrapped 
in  the  clay  mass,  but  which  may  be  removed  by  mechanical 
means,  such  as  filtering,  pressing  and  capillarity. 

Water  of  saturation  is  also  sensible  water  removable  only 
by  evaporation  but  at  normal  temperatures. 

Hygroscopic  water  is  insensible  and  is  removed  only  by 
evaporation  at  temperatures  above  normal. 

Combined  water  is  a  chemical  constituent  of  the  clay  min- 
erals and  can  only  be  driven  off  by  a  destruction  of  the  min- 
eral characteristics. 

How    Drying    Proceeds. 

Clay  is  porous,  and  when  wet,  these  pores  are  filled  with 
water.  The  pores  form  zig-zag  channels  or  capillary  tubes 
from  the  center  of  the  mass  to  the  surface.  When  there  is 
no  drying  at  the  surface,  there  is  practically  no  movement  of 
the  water  in  the  pores — osmotic  movement  not  considered. 
Immediately,  however,  jthat  drying  begins  on  the  surface, 
the  water  thus  evaporated  is  replaced  by  capillarity,  drain- 
ing the  numerous  .minute  reservoirs  which  we  call  pores. 
There  is  no  better  illustration  than  a  lamp  and  wick.  As 
long  as  the  lamp  contains  oil  it  will  be  drawn  up  by  the 
wick  by  capillarity.  When  the  oil  is  exhausted,  the  flow 
ceases,  although  there  is  some  oil  still  clinging  to  the  walls 
of  the  lamp.  Similarly  are  the  pore  reservoirs  in  a  clay  mass 
drained.  This  is  the  first  stage  in  drying  and  the  stage  in 
which  shrinkage  takes  place,  and  if  all  parts  of  the  clay  mass 


It' 


DRYING    CLAY    WARES 


are  being  drained  at  the  same  time,  there  are  no  unequal  or 
local  strains  and  the  ware  does  not  crack. 

It  is  almost  needless  to  say  that  the  safety  of  drying 
during  this  first  stage  depends  upon  the  relative  rate  of 
surface  evaporation  and  replacement. 

If  we  dip  different  substances  in  a  colored  solution  and 
note  the  rise  of  the  solution  in  the  substances,  we  will  have 
a  clear  idea  of  how  in  one  clay  mass,  because  of  its  pore 
channels,  the  included  water  will  move  from  one  place  to 
another  within  the  mass  faster  than  in  another  clay  ,mass 
with  a  different  system  of  pore  channels,  to  replace  water 
removed  by  surface  evaporation. 

Now  if  the  water  is  evaporated  from  the  surface  faster 
than  it  is  replaced,  there  must  be  cracking  to  relieve  the 
strains,  either  that  or  the  wet  core  must  be  compressed 
by  the  dried  and  shrinking  shell,  which  is  very  doubtful.  To 
compress  such  a  core  will  exceed  the  power  of  the  strongest 
press,  and  clay,  however  strong,  has  not  such  strength. 
There  are  undoubtedly  some  strains  introduced.  It  would 
be  folly  to  claim  that  drying  proceeds  absolutely  at  the  same 
rate  throughout  the  mass,  and  whenever  the  strain  exceeds 
the  breaking  strain  of  the  dried  clay,  there  must  be  cracking. 

In  overcoming  minor  strains,  the  physical  character  of 
the  clay  plays  a  part.  Ordinary  clays,  practically  all  clays, 
are  made  up  of  a  number  of  minerals — fragments  of  the  rocks 
from  which  the  clays  were  derived.  If  the  grains  are  angular 
and  coarse,  they  may  be  loosened,  partly  pulled  cut  of  the 
imbedding  matrix,  or  under  compression  will  rearrange  them- 
selves into  greater  compactness,  in  either  case  relieving  the 
relieving  the  strain.  Also  we  must  admit  that  clay  has  some 
elasticity  which  will  relieve  minor  strains. 

The  safe  removal  of  the  moisture  then  depends  upon  three 
factors: 

(1)  The    relative    rate    of    evaporation    and    interstitial 
movement  of  the  moisture. 

(2)  The   shape   and    size   of   the   grains   which    make    up 
the  clay  mass — viz.,  the  interlocking  power. 

(3)  Elasticity. 

Having  removed  the  reservoir  water,  as  above  described, 
there  still  remains  the  moisture  clinging  to  the  walls  of  the 
pore  passages— the  water  of  saturation.  This  can  only  be 
removed  under  the  boiling  point.  Air  will  take  up  and  hold 
orating  the  water  by  direct  contact  with  it. 

This  brings  us  to  the  border  land  of  the  second  stage  of 
the  drying,  or  more  properly,  we  should  make  this  the  second 
step  in  the  first  stage. 


DRYING    CLAY    WARES  11 

When  the  drying  medium  has  penetrated  to  the  core  of 
the  clay  mass,  the  latter  is  bone  dry,  as  we  say,  but  there 
is  still  the  hygroscopic  and  colloidal  water  that  cannot  be 
removed  under  the  boiling  point.  Air  will  take  up  and  hold 
as  vapor  a  certain  amount  of  water  for  each  temperature. 
We  make  this  statement  advisedly.  (Without  confusing  the 
discussion  with  technical  matters,  let  us  say  the  air  has 
absorptive  power  for  the  moisture,  so,  likewise,  has  other 
bodies,  including  clay,  and  which  every  body  has  the  greatest 
absorptive  power,  will  remove  moisture  from  the  other.  We 
may  say  that  the  clay  mass  may  not  become  dryer  than  the 
drying  medium,  and  air  is  never  absolutely  Jry.  We  know 
that  this  statement  is  open  to  attack,  but,  relative  absorptive 
powers  considered,  it  is  true.) 

There  is  then  some  moisture  remaining,  and  especially 
is  this  the  case  when  we  have  lining  the  pore  passages  vari- 
ous salts  common  to  clay  besides  sulphuric  acid.  The  lat- 
ter and  calcium  chloride  are  commonly  used  as  dessicators 
to  remove  moisture  from  the  air,  simply  because  they  have 
greater  affinity  for  moisture  than  air.  Whatever  the  cause, 
some  moisture  remains  in  the  "bone"  dry  clay  mass  which 
can  only  be  removed  by  temperatures  above  the  boiling  point 
of  water,  or  of  the  solution  whatever  it  may  be.  Sulphuric 
acid,  for  instance,  is  only  volatilized  at  680  degrees  Fahren- 
heit. The  removal  of  this  hygroscopic  water  is  the  second 
drying  stage  and  is  done  in  the  kilns — water-smoking. 

We  are  not  concerned  with  chemically  combined  water, 
since  it  is  never  in  the  form  of  moisture,  and  is  only  expelled 
by  the  breaking  up  of  the  kaolin  at  a  temperature  of  800 
degrees  or  higher.  It  correlates  with  the  expulsion  of  carbon- 
dioxide  from  limestone,  or  sulphuric  anhydride  from  sulphates, 
or  sulphur  from  sulphides. 

In  the  water-smoking  stage  of  the  drying  there  is  con- 
siderable danger.  We  may  generate  the  vapor  so  fast 
that  there  is  considerable  pressure,  and  while  it  may  not 
rupture  the  mass  as  a  whole,  yet  it  may  loosen  the  indivi- 
ual  grains  and  thus  cause  a  rather  punky  product  which 
otherwise  would  have  the  steeUike  rjng  so  much  desired. 
The  grains  are  loosened  in  their  sockets  in  the  matrix  and 
while  they  may  not  pull  out  entirely,  yet  they  cannot  con- 
duct the  vibratory  motion  which  is  essential  if  the  ware  is 
to  ring  like  steel.  A  cracked  bell  or  a  cracked  vase  will 
not  ring  true,  neither  will  a  brick  ring  true  that  has  an 
infinite  number  of  minute  cracks,  or  loosened  grains.  This 
is  why  steamed  bricks  are  so  often  rotten. 


DRYING    CLAY    WARES 


CHAPTER  II. 
The  Work  to  Be  Done. 

WE  HAVE  SHOWN  how  drying  proceeds.  It  will  be  in 
order  next  to  determine  the  work  to  be  done  and  the 
heat  required. 

Clays  vary  in  the  amount  of  water  they  will  take  up 
in  order  to  make  them  plastic.  The  following  table,  made 
up  from  various  sources,  gives  the  percentage  of  water  re- 
quired for  the  clays  in  question: 

Missouri  Geological  Survey. 

Brick  clay   16 — 19  per  cent. 

Fire  and  potters'  clay.  .....  .15 — 33  per  cent. 

Flint  clay   15 — 24  per  cent. 

Kaolins   18 — 35  per  cent. 

Shales .  14 — 25  per  cent. 

Georgia   Geological   Survey. 

Brick  clays    1 5—30  per  cent. 

Kaolins 30 — 45  per  cent, 

Fuller's  earth  up  to 90  per  cent. 

North   Carolina  Geological   Survey. 

All  clays  16—40  per  cent. 

Illinois   Geological   Survey. 

Paving  brick  clays 13—17  per  cent. 

Other  clays   15—21  per  cent. 

Oklahoma   Geological    Survey. 

Shale  ..  ..10—32  Average  22.5  per  cent. 

Light  burning  clays,  "         25.1  per  cent. 

Fir6  clay«   "         15.1  per  cent 

Surface  clays,  15-36  "         24.9  per  cent. 

The  amount  of  water  in  clay  ware  depends  upon  the  pro- 
>s   as   well   as   the   clay.      The   dry   or    dust-pressed   wares 


DRYING    CLAY    WARES  13 

have  very  little  water.  In  the  semi-dry  processes,  the  clay 
is  damp  enough  to  ball  up  in  the  hand,  and  this  is  the  usual 
condition  of  the  clay  for  dry-pressed  bricks. 

Stiff  mud  processes  require  more  water — from  10  to  25 
per  cent,  of  the  dry  clay.  The  soft  mud  has  a  still  higher 
per  cent.,  and  casting  processes  require  that  the  clay  be 
made  up  into  a  thin  slip,  but  as  this  is  absorbed  by  the 
plaster  moulds,  the  ware  itself  has  scarcely  more  water  than 
would  be  required  for  soft  mud. 

In  the  several  processes,  but  especially  in  the  stiff  mud, 
the  softness  of  the  ware  has  a  wide  range.  Some  clays  can 
only  be  made  into  stiff  mud  wares  by  working  them  so  soft 
that  they  easily  dent  by  the  fingers  in  handling  them,  and 
paddles  or  hand  clamps  are  necessary  for  the  handling. 
Other  stiff  mud  wares  are  so  hard  that  in  bricks,  for  instance, 
they  may  be  hacked  ten  to  fifteen  courses  high.  In  several 
plants,  for  a  number  of  years,  stiff  mud  bricks  have  been 
set  in  the  kiln  six  to  ten  courses  high  and  dried  therein. 
Experiments  are  being  made,  having  in  view  setting  the 
bricks  eighteen  to  twenty  courses  high  in  the  kiln,  making 
this  the  maximum  height  of  the  setting  and  drying  in  the 
kiln  followed  by  burning,  thus  eliminating  the  dryer  just 
as  in  the  dry-pressed  bricks.  Clays  which  will  stand  this 
setting  must  be  suitable,  and  naturally  will  require  little 
water. 

In  our  discussion  of  the  work  of  drying,  we  will  assume 
that  a  standard  sized  brick  weighing  five  and  one-half  pounds 
burned,  will  contain  one  pound  of  free  water.  This  may 
seem  low,  but  it  must  be  remembered  that  there  are  three 
water  conditions  that  enter  into  loss  in  weight  in  burning 
— chemical  water,  hygroscopic  water,  and  moisture.  In  dry- 
ing we  are  only  concerned  with  the  moisture. 

Many  people  get  the  idea  that  because  clay  wares  can 
be  dried  readily  at  normal  temperatures  in  the  open  air, 
that  very  little  heat  is  required  for  the  drying.  We  wish 
to  state  emphatically  that  regardless  of  temperature,  a  cer- 
tain number  of  heat  units  are  required  to  vaporize  water, 
and  this  heat  must  be  supplied  from  same  source. 

(Note — We  wish  to  encumber  this  discussion  with  tech- 
nicalities as  little  as  possible,  and  make  this  note  to  avoid 
the  criticism  of  some  technicist  who  concerns  himself  chiefly 
with  technicalities.  Water  vaporizes  at  all  practical  tem- 
peratures. It  will  take  less  heat  to  vaporize  water  at  60 


DRYING    CLAY    WARES 


degrees  Fahrenheit  than  212  degrees  or  325  degrees.     First, 
the  water  is  only   heated   to   60   degrees   instead   of   ^ 
grees  or  325  degrees.     This  is  the  sensible  heat.     Second,  it 
requires  more  heat  to  maintain  the  pressure,  the  greater  the 
pressure.) 

We  are  accustomed  to  say  that  air  absorbs  the  vapor,  and 
will  continue  to  use  that  term.  Water  will  evaporate  at  any 
temperature  until  the  requisite  vapor  pressure  for  that  tem- 
perature is  reached,  and  this  takes  place  in  a  vacuum  prac- 
tically as  well  as  in  air  or  in  any  gas  mixture. 

We  readily  appreciate  this  in  boiler  practice,  where  the 
temperature  of  the  water  must  be  advanced  with  every  ad- 
vance in  gauge  pressure.  The  same  is  true  of  temper- 
atures below  212  degrees,  only  here  the  boiler  is  not  in 
evidence,  but  the  pressure  is  there  just  the  same.  We  say 
the  air  is  saturated,  meaning  the  vapor  pressure  is  satisfied. 

For  the  benefit  of  those  who  wish  to  figure  these  ques- 
tions closely,  we  insert  the  following  table,  giving  the  heat 
units  of  vaporization. 

32  degrees— 1092  B.  T.  U. 

60  degrees— 1100  B.  T.  U. 
100  degrees— 1112  B.  T.  U. 
212  degrees— 1147  B.  T.  U. 

307  degrees — 1176  B.  T.  U.  60  pounds  pressure. 
324  degrees — 1181  B.  T.  U.  80  pounds  pressure. 
338  degrees — 1185  B.  T.  U.  100  pounds  pressure. 
350  degrees— 1189  B.  T.  U.  120  pounds  pressure. 

We  will  make  use  of  this  table  in  discussing  steam 
dryers. 

To  vaporize  a  pound  of  water  at  212  degrees  Fahrenheit, 
at  sea  level,  967  B.  T.  U.  are  required.  This  is  the  heat  that 
disappears — becomes  latent.  It  represents  the  heat  required 
to  keep  the  pot  boiling  without  any  advance  in  temperature 
of  the  water  in  the  pot.  One  hundred  and  eighty  B.  T.  U. 
per  pound  of  water  are  required  to  bring  the  water  up  to 
the  boiling  point  from  32  degrees  Fahrenheit,  or  140  B.  T. 
U.  to  advance  the  temperature  from  72  degrees  to  boiling. 
Add  this  140  to  967  and  we  have  1,107  B.  T.  U.  to  vaporize 
a  pound  of  water  at  212  degrees  Fahrenheit  from  72  degrees 
Fahrenheit.  Let  us  say  1,100  B.  T.  U. 

We  must  generate  this  heat  or  rob  the  atmosphere  to 
the  extent  of  the  latent  heat  for  every  pound  of  water  we 
evaporate  from  our  wares. 


DRYING    CLAY    WARES  15 

A  pound  of  coal  may  have  from  8,000  to  15,000  (averaps- 
12,000)  B.  T.  U.,  and  nearly  one  pound  of  coal  is  required  to 
evaporate  ten  pounds  of  water.  Ten  bricks  then  require  one 
pound  of  coal;  a  thousand  bricks  require  100  pounds  of  coal. 

But  this  is  only  part  of  the  work.  We  put  in  6,000  pounds 
of  clay  (1,000  bricks),  at  60  degrees  Fahrenheit  and  take 
them  out  at  130  degrees  Fahrenheit  perhaps.  This  takes 
84,000  B.  T.  U.  [6,000 X. 2  (sp.  ht.  of  clay )X 70,]  or  seven 
pounds  of  coal.  We  put  in  800  pounds  of  iron  cars  per  thou- 
sand bricks,  which  requires  6,260  B.  T.  U.  (800X. 11X70), 
or  one-half  pound  of  coal. 

We  may  use  720,000  cubic  feet  of  air  in  drying  1,000  bricks, 
or  57,600  pounds,  which  requires  967,670  B.  T.  U.  (57,600 X. 24 X 
70)  or  80  pounds  of  coal. 

Now  we  have  in  round  numbers  188  pounds  of  coal.  If 
the  radiation  loss  is  ten  per  cent.,  the  total  fuel  requirement 
is  206  pounds,  or  practically  one-tenth  ton  of  coal  to  dry 
1,000  bricks.* 

These  figures  will  be  surprising  to  many,  but  there  are 
many  dryers  using  a  greater  quantity.  In  fact,  a  half  more 
is  common  practice,  and  double  is  not  infrequent.  Our  figures 
are  on  the  assumption  that  we  get  all  the  heat  from  the  fuel 
into  the  dryer. 

If  we  are  drying  with  steam,  we  must  introduce  the  boiler 
losses. 

If  we  use  direct  heat,  we  must  allow  for  the  loss  in  the 
products  of  combustion,  for  imperfect  combustion,  either 
through  too  much  or  too  little  air,  loss  in  ash,  etc.  How- 
ever we  may  do  the  work,  there  is  an  absorption  of  heat,  and 
the  fuel  required  to  generate  this  heat  will  in  some  factories 
exceed  the  fuel  used  in  burning  the  ware. 

The  fuel  consumption  in  burning  is  frequently  discussed 
in  conventions,  but  we  seldom  hear  any  mention  of  the  fuel 
used  in  drying. 

The  owners  of  open  yards  will  often  view  with  envy  the 
modern  drying  plant  of  their  competitors,  its  less  labor,  per- 
haps less  drying  loss,  but  if  they  could  appreciate  the  fact 
that  the  gain  is  at  an  expense  of  ten  to  fifteen  dollars  per 
day  in  fuel,  they  would  be  more  content  with  their  old-fash- 
ioned process. 


*Note — The  above  figures  are  general,  but  approximately 
correct  for  a  direct  fired  dryer.  The  heat  consumption  will 
vary  with  the  method  of  application  and  will  be  considered 
in  detail  in  the  discussion  of  the  several  types  of  dryers. 


DRYING    CLAY    WARES 


CHAPTER  III. 
The  Relation  of  Air  to  Drying. 

WE  HAVE  USED  the  expression,  "the  air  absorbs 
moisture,"  and  yet  stated  that  the  air  plays  no 
part  in  absorption.  We  wish  to  set  ourselves  right 
in  this  matter  and  have  a  clear  understanding  of  it. 

Scientists  hold  that  water  as  vapor  is  absorbed  by  air,  that 
it  goes  into  solution  just  as  salt  may  go  into  solution  in  water. 
Such  problems  in  physical  chemistry  do  not  concern  us. 

From  our  standpoint  of  drying  clay  wares,  we  may  con- 
sider that  air  does  not  absorb  moisture  because  air  is  not  an 
essential  factor  in  vaporization. 

If  we  could  inclose  a  cubic  foot  of  air  saturated  with 
moisture  and  then  could  remove  the  air,  the  moisture  would 
remain  suspended  in  the  space  as  vapor. 

If  we  fill  a  long  tube  closed  at  one  end,  with  mercury, 
and  invert  it  in  a  mercury  bath,  the  mercury  in  the  tube 
will  drop  to  about  29"  or  barometric  pressure.  The  space 
above  the  mercury  in  the  tube  is  a  vacuum.  Now  suppose 
we  introduce  a  drop  of  water  in  the  tube;  the  mercury  will 
quickly  drop,  and  this  fall  is  not  due  to  the  weight  of  the 
water,  which  is  insignificant,  but  to  the  vapor  from  the  water 
which  fills  the  upper  part  of  the  tube  and  which  exerts  a 
pressure  upon  the  mercury  and  depresses  it,  or  in  other 
words,  partially  overcomes  atmospheric  pressure,  which  sup- 
ports the  colujnn  of  mercury.  If  all  the  water  passes  into 
vapor,  the  saturation  of  the  vacuum  may  not  be  complete, 
and  the  introduction  of  ,more  water  will  cause  further  de- 
pression of  the  mercury,  but  finally  a  point  will  be  reached 
when  saturation  is  complete  and  no  more  water  will  evap- 
orate, and  no  further  change  will  take  place  in  the  mercury 


DRYING    CLAY    WARES  17 

level.  Suppose  we  now  introduce  some  other  liquid.  The 
space  is  saturated  with  water  vapor,  but  not  with  the  vapor  of 
this  other  liquid,  and  the  latter  evaporates  and  the  mercury 
level  is  still  further  depressed.  The  pressure  exerted  by  these 
vapors  is  called  vapor  pressure.  If  we  cool  the  tube  some  of 
the  vapor  condenses  and  the  mercury  rises,  but  if  we  raise 
the  temperature,  we  increase  the  vapor  pressure  (volume  of 
liquid  vaporized),  and  the  mercury  falls.  If  we  introduce  air, 
the  mercury  still  further  falls,  and  additional  vapor  is  taken 
up  in  consequence  of  the  greater  space,  and  perhaps  in  con- 
sequence of  the  introduction  of  the  air. 

Suppose  we  could  take  a  cubic  foot  of  saturated  air  from  a 
dryer  at  60  degrees,  and  remove  the  moisture  by  dessicators, 
by  cooling,  or  in  any  way,  we  would  have  a  cubic  foot  of  rare- 
fied air,  but  less  than  a  cubic  foot  at  atmospheric  pressure. 
Repeat  the  experiment  at  120  degrees,  and  our  remaining 
air  will  occupy  a  much  smaller  volume,  and  at  212  degrees 
there  may  be  no  air  whatever. 

In  other  words,  while  air  in  a  closed  space  does  not  inter- 
fere with  the  weight  of  moisture  taken  up,  and  indeed  may  be 
in  some  slight  degree  an  aid,  yet  in  free  space  the  water  vapor 
displaces  air  and  at  the  boiling  point  or  above  the  air  may  be 
entirely  driven  out. 

From  this  it  will  be  seen  that  practically  air  has  nothing 
to  do  with  evaporation.  Evaporation  is  a  function  of  vapor 
pressure,  and  vapor  pressure  is  influenced  by  temperature. 

The  part  air  plays  is  mechanical,  and  very  important. 
Referring  again  to  the  mercury  tube,  suppose  we  have  a 
stopcock  in  the  top  of  the  tube  opening  into  a  larger  vacuum. 
When  the  mercury  has  reached  its  lowest  level,  due  to  the 
vapor  pressure,  if  we  open  the  stopcock  the  mercury  will 
rise  and  force  out  the  vapor.  Closing  the  stopcock  repro- 
duces the  former  conditions  and  the  water  will  evaporate 
as  before  and  force  down  the  mercury.  With  air  we  can 
create  a  draft  and  sweep  away  the  vapor  as  fast  as  it  is 
taken  up  by  space.  If  it  were  not  removed,  drying  would 
cease  when  the  vapor  pressure  was  attained.  As  the  tem- 
perature rises,  the  vapor  pressure  increases,  and  at  212  de- 
grees equals  the  pressure  of  the  atmosphere.  We  now  no 
longer  need  any  air  because  the  steam  will  overcome  at- 
mospheric pressure  and  forces  itself  out,  not  because  of  the 
air,  but  in  spite  of  it.  Steam  will  rush  out  of  a  boiler  with 
great  force,  pushing  the  air  away. 


18  DRYING    CLAY    WARES 

If  we  are  working  at  a  low  temperature,  we  must  intro- 
duce a  large  volume  of  air  because  each  cubic  foot  has  space 
for  only  so  much  vapor  and  must  be  removed  when  saturated 
if  the  drying  is  to  continue.  As  we  increase  the  temper- 
ature, the  capacity  to  take  up  moisture  increases,  and  we 
need  less  volume  to  carry  it  away.  Above  212  degrees  we 
theoretically  need  no  air  whatever,  but  in  practice  it  serves 
as  a  convenient  medium  to  conduct  the  heat  from  the  source 
to  the  ware.  We  say  convenient — not  essential. 

The  question  may  arise,  does  not  the  volume  of  air  in- 
crease with  temperature  in  equal  ratio  with  the  vapor  pres- 
sure, and  if  so,  will  there  not  be  required  the  same  volume  of 
air  for  all  temperatures? 

Let  us  glance  at  this. 

The  following  table  from  Seger's  formula  for  vapor  ca- 
pacities has  been  especially  calculated  for  this  paper. 
Seger's  formula  is: 

p      .623 

V  =  1.293  X — X —    =;  wt.  of  vapor  in  kilos  per  cu.  meter  of  air 
760  1+at 

1.293  =  wt.  of  1  cu.  meter  or  dry  air  at  temperature  t. 

p  =  tension  of  vapor  at  temperature  t. 

a  =  0.00366  =  co-efficient  of  expansion  of  gas. 
t  =  temperature. 
0.623  =  specific  weight  of  water  vapor  where  dry  air  equals  1. 

We  have  changed  the  formula  to  pounds,  Fahrenheit,  etc., 
and  we  have  assumed  an  altitude  of  800'  or  29"  of  mercury 
instead  of  760  millimeters. 

Our  formula  becomes: 

P'  0.623 

V=1.293X  0.062  X-  -X100=wt.  of  j     lb 

29     l+0.002(t'-32) 

per  100  cu.  ft.  of  air 

The  column  on  the  left  gives  the  value  of  p'  for  the  sev- 
eral temperatures.  In  the  table  itself,  the  first  column  gives 
the  temperature;  the  second  column  is  for  100  per  cent  or 
complete  saturation;  the  third  column  is  90  per  cent  satura- 
lon,  etc.  See  table  on  opposite  page. 

Now   as   seen   in   the   second   column,   in    advancing   from 
degrees   to   200    degrees,    the    capacity    for   moisture    in- 
ten  times~namely'  from  0-292  pounds  to 
aCC°rdinS  ^   the   ratio   1+ 


DRYING    CLAY    WARES 


This  is  an  increase  of  17  per  cent,  in  the  volume  of  air,  or 
rather  the  air  is  rarefied  to  this  extent  while  the  carrying 
capacity  of  equal  volumes  has  increased  over  1,000  per  cent. 
About  one-eighth  of  the  air  volume  is  required  at  the  higher 
temperature  to  do  the  same  work. 

We  will  have  occasion  to  refer  to  the  above  table  in  the 
discussion  of  some  of  the  types  of  dryers. 

Having  explained  the  relation  of  air  to  vaporization,  we 

VAPOR    CAPACITY    OF   100   CU.   FT.    OF    AIR. 


Deg. 

Percentage  of  Saturation 

p' 

Fahr. 

100 

90 

80 

70 

60 

50 

40 

0.361. 

.    50 

0.060 

0.054 

0.048 

0.042 

0.036 

0.030 

0.024 

0.518. 

.   60- 

0.085 

0.077 

0.068 

0.059 

0.051 

0.043 

0.034 

0.733. 

.   70 

0.117 

0.105 

0.094 

0.070 

0.059 

0.047 

1.024. 

.   80- 

0.161 

0.145 

0.129 

0.113 

0.097 

0.081 

0.064 

1.410. 

.    90 

0.218 

0.196 

0.174 

0.153 

0.131 

0.109 

0.087 

1.918. 

.100- 

0.292 

0.263 

0.233 

0.204 

0.175 

0.146 

0.117 

2.578. 

.110 

0.384 

0.346 

0.307 

0.269 

0.230 

0.192 

0.154 

3.425. 

.120- 

0.502 

0.452 

0.402 

0.351 

0.301 

0.251 

0.201 

4.503. 

.130 

0.650 

0.585 

0.520 

0.455 

0.390 

0.325 

0.260 

5.859. 

.140- 

0.830 

0.747 

0.664 

0.581 

0.498 

0.415 

0.332 

7.545. 

.150 

1.051 

0.946 

0.841 

0.736 

0.631 

0.626 

0.421 

9.628. 

.160- 

1.320 

1.188 

.056 

0.924 

0.792 

0.660 

0.528 

10.850. 

.165 

1.476 

1.328 

.181 

1.033 

0.886 

0.738 

0.590 

12.180. 

.170 

1.644 

1.480 

.315 

1.161 

0.986 

0.822 

0.658 

;.;  |M 

.17.-, 

1.826 

1.643 

.461 

1.278 

1.096 

0.913 

0.730 

15.270. 

.180- 

2.029 

1.826 

.623 

1.420 

1.217 

1.015 

0.812 

17.060. 

.185 

2.250 

2.025 

800 

1.576 

1.350 

1.125 

0.900 

19.000. 

.190 

2.486 

2.237 

1.989 

1.740 

1.492 

1.243 

0.994 

20.260. 

.  1!.:, 

2.631 

2.368 

2.105 

1.842 

1.579 

1.316 

1.052 

23.460. 

.200- 

3.024 

2.722 

2.419 

2.117 

1.814 

1.512 

1.209 

shall  continue  to  use  the  term  "moisture  absorbed  by  the  air," 
rather  than  "moisture  in  space." 

Effect  of   Lamination   on   Drying. 

Lamination,  as  we  know,  is  due  to  the  slipping  of  the  clay 
on  itself,  forming  planes  or  "slickensides"  in  the  clay  mass. 

We  can  hardly  conceive  a  porous  plastic  mass  being 
slipped  on  itself  without  closing  up  the  pores.  Where  such 
closing  up  takes  place  the  effect  in  drying  is  serious. 

We  have  said  that  safe  drying  can  only  take  place  when 
the  water  in  the  mass  is  brought  to  the  surface  as  fast  as 
evaporation  on  the  surface  takes  place. 

Now  if  the  clay  mass  is  made  up  of  a  series  of  plates 
as  would  be  the  case  when  laminated,  the  passages  for  the 
water  are  broken  and  closed  along  the  planes  of  the  lamina- 


DRYING    CLAY    WARES 


tions  In  order  to  dry  such  masses  safely,  the  rate  of  evap- 
oration should  only  be  the  same  as  the  rate  of  the  passage 
of  the  water  across  the  laminated  planes.  Suppose  the  evap- 
oration rate  should  he  faster  than  this,  let  us  say  at  a  rate 
which  would  be  safe  if  the  clay  were  not  laminated,  the  wate 
in  the  outer  shell  is  drawn  to  the  surface  at  a  faster  rate 
than  the  water  can  get  into  this  shell  from  the  inner  core. 
The  outer  shell  becomes  leather  hard  and  finally  bone 
dry  In  becoming  so,  it  must  shrink  or  crack,  and  it  naturally 
cracks  The  tendency  to  shrink  and  the  cracking  causes  the 
shell  to  creep  more  or  less  on  the  core  and  breaks  whatever 
bond  there  may  have  been. 

The  air  enters  the  cracks  and  begins  work  upon  the  sec- 
ond shell  and  the  result  is  duplicated  and  a  second  ring  or 
shell  is  dried,  cracked  and  loosened  from  the  core.  The  final 
dried  mass  is  a  series  of  cracked  and  loosened  concentric 
shells,  or,  as  we  say,  badly  shattered.  In  order  to  make  such 
a  clay  safe  drying,  or  to  get  a  solid  product  from  it,  the  first 
step  is  to  overcome  lamination. 

Grog. 

Lamination  is  often  discussed  and  \ve  will  not  enter  into 
it  except  as  it  relates  to  drying. 

The  usual  remedies  for  lamination  are  lubrication,  grog, 
lamination  bars,  etc.  Grog  plays  quite  a  part  in  drying  as 
well  as  in  lamination.  It  reduces  lamination  si,mply  because 
of  its  granular  character.  It  acts  as  a  binder,  as  a  lot  of 
teeth,  as  a  drag  to  prevent  the  clay  slipping  on  itself.  To 
serve  this  purpose  it  must  be  relatively  coarse  and  angular. 
Its  effect  on  drying  is  first  to  increase  the  pore  space  so  the 
water  will  flow  faster  to  the  surface;  second,  to  reduce  shrink- 
age, thus  reducing  the  degree  of  strains;  third,  it  increases 
the  strength  of  the  clay  because  of  its  binding  action  and  en- 
ables the  clay  to  withstand  the  strains.  The  coarser  and  more 
angular  the  grog,  the  better  it  serves  as  a  binder,  and  the 
larger  the  pore  spaces. 

Sand,  which  is  most  commonly  used  as  a  grog  because  it  is 
most  available,  is  far  from  the  best  material.  It  is  deposited 
from  the  water,  and  in  consequence  it  is  made  up  of  rounded 
instead  of  angular  grains,  through  the  rolling  and  tumbling 
it  gets  from  its  source  to  the  final  deposit.  The  density  and 
smoothness  of  the  surface  of  the  grains  do  not  permit  the 
plastic  clay  to  cling  as  closely  to  it  as  to  a  rougher  and  more 


DRYING    CLAY    WARES  21 

porous  material.  Finally  in  the  burning  there  can  be  no  bond 
between  the  clay  and  the  sand  except  at  high  tejnperature, 
but  this  does  not  concern  us  in  the  drying  problem. 

Crushed  quartz  or  quartzite  is  a  better  material  than 
sand  because  of  its  angular  character,  but  this  is  available 
in  very  few  yards. 

The  best  grog  is  crushed  burned  clay,  and  often  there  is 
enough  waste  about  the  plant  to  make  sufficient  grog.  An- 
other good  material  is  crushed  clinkers  from  the  kiln  and 
boiler  furnaces,  but  it  can  only  be  used  in  common  wares. 

Such  materials  have  the  advantage  of  being  rough  and 
angular,  and  in  drying  have  the  further  advantage  of  being 
porous.  If  we  have  lamination  planes  bound  together  with 
a  lot  of  porous  grog,  the  water  can  get  across  through  the 
binding  material  if  it  cannot  find  its  way  through  the  inter- 
stitial spaces. 

The  porous  grog,  because  of  its  pores  acting  as  a  lot  of 
suckers,  draws  the  plastic  clay  into  close  contact  and  forms  a 
much  better  bond  than  could  exist  between  clay  and  sand. 

Where  sand  suffices  nothing  further  need  be  said,  but 
where  sand  fails,  as  it  often  does,  it  is  well  to  know  that 
there  is  a  much  better  material. 

Preheating  Clay. 

Prof.  A.  V.  Bleininger  deserves  great  credit  for  his  work 
on  preheating  clays  as  a  means  of  overcoming  drying  trou- 
bles. Preheating  a  clay  .makes  it  more  porous  and  permits 
the  moisture  to  escape  to  the  surface. 

The  peculiar  condition  of  many  clays  which  makes  them 
difficult  to  dry  is  the  subject  of  much  study  and  discussion 
at  the  present  time  and  brings  us  to  the  borderland  of  our 
knowledge.  The  colloid  theory  is  now  generally  accepted  by 
clay  technicists.  Briefly,  we  assume  that  the  clay  contains 
a  great  number  of  cells  or  sacs  which  absorb  water  through 
their  walls,  swell  and  close  up  the  interstitial  pore  spaces. 
This  puts  a  stop  to  any  flow  of  water  to  the  surface,  and 
when  the  surface  dries,  it  must  crack  to  relieve  the  strain. 
The  air  gets  into  the  cracks  and  continues  the  drying  pro- 
cess which  at  the  same  time  deepens  the  cracks.  The  drying 
cracks  in  a  colloidal  clay  are  irregularly  hexagonal  in  shape 
and  are  characteristic. 

The  preheating  bursts  the  sacs  and  sets  free  the  in- 
cluded water,  driving  it  off,  and  at  the  same  time  so  destroys 


DRYING    CLAY    WARES 


the  structure  of  the  sac  that  it  cannot  take  up  water  and  hold 
it  except  by  surface  tension  which  applies  in  any  case,  nor 
can  the  colloids  now  swell  and  close  up  the  pores  in  the  clay 

Clayworkers  have  been  slow  to  take  up  preheating,  and 
there  may  arise  a  number  of  practical  difficulties. 

The  temperature  must  be  carried  considerably  above  the 
drying  degree,  and  it  remains  to  be  proven  whether  we  have 
a  practical  preheating  range,  otherwise  it  becomes  of  ques- 
tionable value.  Clays  differ  in  the  degree  of  preheating  re- 
quired, and  the  variation  ranges  from  250  degrees  C.  (450 
degrees  F.)  to  450  degrees  C.  (810  degrees  F.).  Suppose 
a  clay  is  not  sufficiently  heated  at  500  degrees  Fahrenheit 
and  tco  much  plasticity  is  lost  at  600  degrees  Fahrenheit, 
then  we  must  keep  within  this  range,  else  we  will  get  some 
cracked  ware  in  drying  on  one  extreme  and  some  loss 
through  weak  bonding  on  the  other  extreme.  Therein  lies  the 
uncertainty  of  preheating— the  range  may  be  less  than  we 
can  work  within. 

In  testing  one  material,  we  found  that  the  addition 
of  burned  clay  grog  to  the  raw  clay  served  the  same  purpose 
as  preheating,  and,  if  it  applies  in  all  cases,  will  greatly  sim- 
plify the  problem. 

It  is  analogous  to  the  trouble  with  the  Bessemer  con- 
verter in  making  steel.  Originally,  it  was  intended  to  stop 
the  process  when  the  impurities  in  the  metal  had  been 
burned  out  to  a  required  degree,  but  it  was  impossible  to 
stop  at  the  right  point,  and  successive  blows  differed  widely 
in  the  character  of  the  .metal.  The  process  became  success- 
ful when  the  everburning  was  resorted  to  and  the  over- 
burned  metal  corrected  by  the  addition  of  a  reducing  element 
which  could  be  added  in  definite  proportions. 

We  may  not  be  able  to  preheat  clay  to  the  proper  de- 
gree of  uniformity,  but  if  grog  will  serve  the  same  purpose, 
we  can  add  it  in  any  determined  amount.  We  think  that 
the  part  that  burned  clay  grog  plays  is  not  only  that  of 
opening  up  the  structure  mechanically,  but,  by  the  absorp- 
tive power  of  its  pores,  it  will  draw  the  water  from  the  col- 
loids, and  collapse  the  cells  which  clog  the  pores  of  the  mass. 


DRYING    CLAY    WARES  23 


CHAPTER  IV. 
Shrinkage. 

BEFORE  TAKING  up  a  description  and  discussion  of 
dryers,  we  wish  to  consider  the  question  of  shrinkage. 
\Vhy  do  wet  clays  shrink?  It  seems  almost  a  foolish 
question,  but  if  we  could  properly  answer  it  we  would  un- 
derstand the  cause  of  our  drying  trouble  and  perhaps  more 
easily  eliminate  it. 

What  is  shrinkage?  The  answer  is  easy.  It  is  a  drawing 
together  of  the  particles  of  clay  toward  a  common  center, 
each  particle  pulling  the  next  beyond.  When  the  chain  of 
particles  is  too  long,  it  breaks,  causing  a  crack  in  the  ware. 
Every  break  establishes  a  new  center  with  a  shorter  chain  of 
particles  and  we  have  safe  drying  only  when  the  mass  around 
each  center  is  proportionate  to  the  forces  pulling  the  particles 
together. 

There  are  several  forces  at  work  though  they  all  may  be 
grouped  under  the  head  of  gravitation.  It  is  the  attraction  of 
one  jnolecule  for  another  in  the  same  material  (cohesion),  of 
a  molecule  in  one  material  for  a  molecule  in  a  different  ma- 
terial (adhesion),  of  differential  cohesion  (surface  tension),  of 
adhesion  in  minute  passages  (capillarity),  that  cause  a  clay 
to  shrink. 

The  surface  of  a  liquid  is  stronger  and  more  difficult  to 
rupture  than  within  the  liquid.  It  is,  as  it  were,  an  elastic 
skin  or  coating,  which  will  support  bodies  that  will  not  float 
once  the  surface  is  broken  and  the  body  submerged.  A 
needle  may  be  floated  on  water,  but  sinks  immediately  when 
submerged.  This  surface  force,  or  skin  to  retain  the  sUnile, 
is  called  surface  tension.  In  any  liquid  each  molecule  attracts 
all  surrounding  molecules.  It  is  pulling  and  being  pulled  in 
every  direction  and  may  be  said  to  be  in  equilibrium.  Being 
balanced,  little  force  is  required  to  start  it  in  motion.  At  the 
surface,  however,  there  are  no  .molecules  above,  and  in  con- 
sequence the  surface  molecules  are  not  balanced.  They  are 
held  down  by  the  attraction  between  them  and  the  molecules 


24  DRYING    CLAY    WARES 


below,  and  it  requires  more  force  to  move  them  than  the  sub- 
merged molecules.     Thus  we  explain  surface  tension. 

Citing  again  the  floating  needle,  it  remains  on  the  surface 
because  the  only  forces  acting  are  terrestrial  gravity  pulling 
the  needle  down  and  surface  tension  resisting  this  pull.  The 
water  is  depressed  under  the  needle,  and  there  is  a  stretching 
apart  of  the  surface  molecules,  but  not  beyond  the  limit  of 
elasticity,  or  more  properly,  not  sufficient  to  overcome  the 
forces  of  surface  tension. 

Iron  is  nearly  eight  times  as  heavy  as  water,  and  it  is  evi- 
dent that  surface  tension  which  supports  such  a  weight  is  a 
force  worthy  of  consideration.  The  needle  must  be  oiled  or 
waxed  to  make  it  float.  There  must  be  no  adhesion,  because 
if  the  water  can  stick  to  the  needle  it  will  immediately  begin 
to  climb  by  capillarity,  and  we  have  this  force  added  to  grav- 
ity to  pull  the  needle  down. 

Capillarity  is  a  much  greater  force  than  gravity.  Witness 
the  sap  rising  in  the  trees  to  heights  of  three  hundred  feet  or 
more  (sequoia  trees)  directly  or  indirectly  by  the  force  of 
capillarity  acting  against  gravity  and  overcoming  friction. 
Here  we  have  a  force  of  ten  atmospheres,  with  apparently  no 
decrease  in  the  acting  force.  Again  if  a  porous  body  is 
placed  in  a  closed  vessel  and  submerged  in  water,  capillarity 
will  take  up  the  water  and  drive  out  the  air  from  the  body 
to  develop  in  the  vessel  a  pressure  of  four  to  five  atmos- 
pheres. 

Who  knows  the  power  of  capillarity? 

Of  course,  capillarity  is  merely  an  application  of  adhesion, 
and  the  latter  is  a  force  inestimable.  Government  tests  show 
that  nearly  50  per  cent,  of  the  water  in  a  clay  is  retained 
against  a  force  three  thousand  times  the  force  of  gravity. 

We  do  not  appreciate  the  power  of  these  forces  in  connec- 
tion with  time. 

We  can  tear  a  mass  of  wet  clay  apart  with  our  hands,  but 
no  mechanical  power  can  pull  the  water  loose  from  the  clay 
grains.  We  may  tear  the  .mass  apart,  but  how  much  power 
will  be  required  to  compress  the  mass  to  the  degree  of  natural 
shrinkage?  But.  you  will  say.  the  mass  is  full  of  water  and 
water  cannot  be  appreciably  compressed.  True,  but  if  there 
were  no  clay,  the  water  would  run  out  the  smallest  orifice 
by  the  force  of  gravity  alone.  In  a  mixture  of  water  and  wax 
>r  water  and  mercury,  or  any  mixture  in  which  there  is  no 
adhesion  and  consequently  no  capillarity,  we  could  easily 


DRYING    CLAY    WARES  25 

squeeze  out  the  water;  but  in  clay  it  is  impossible,  except  in 
small  degree. 

Adhesion,  cohesion,  capillarity  and  surface  tension — these 
are  the  forces  that  cause  shrinkage,  and  if  we  give  them  the 
proper  assistance  they  will  do  their  work  faithfully.  The 
trouble  is  that  we  ask  too  much  of  them.  We  set  one  force 
against  another,  and  in  the  equilibrium  which  follows  our  ware 
ruptures. 

Let  us  consider  a  clay  body.  In  a  slip  prepared  for  dipping 
or  casting  the  clay  grains  are  in  suspension  in  the  water.  The 
larger  or  coarser  particles  settle  quickly,  but  the  finer  grains 
may  float  for  days.  Here  we  have  widely  separated  grains  of 
clay  in  a  water  matrix.  They  are  moving  about,  but  hardly 
can  come  in  contact.  As  two  particles  approach,  the  si>ace 
between  decreases,  capillarity  increases,  and  a  current  of 
water  is  drawn  up  between  them  driving  them  apart.  If  two 
plates  of  glass  are  placed  on  edge  in  a  shallow  vessel  of  water 
and  held  like  a  slightly  opened  book,  it  will  be  noticed  that  the 
water  will  rise  highest  at  the  hinge  or  back  edge  and  will  drop 
in  a  curve  to  the  natural  water  level  toward  the  open  edges. 
Closing  the  angle  sends  the  curve  upward,  opening  causes  it 
to  drop.  Similarly  when  grains  of  floating  clay  approach  or 
fall  apart,  the  currents  set  in  motion  counteract  the  move- 
.ment. 

A  bed  of  quicksand  looks  solid,  but  the  grains  are  quies- 
cently afloat  in  water.  Puddle  the  bed  in  the  slightest  degree, 
and  we  set  the  grains  of  sand  in  motion  in  their  watery  beds. 
A  beach  sand,  on  the  contrary,  is  solid,  and  one  may  follow 
the  waves  out,  scarcely  leaving  a  footprint  on  the  wet  sand. 
The  difference  is  due  to  size  of  grain  and  surface  area.  We 
may  float  a  needle  on  water,  but  not  an  iron  shot,  though 
they  have  the  same  weight. 

In  clays  we  have  a  large  percentage  of  very  fine  material 
(some  authorities  hold  that  plasticity  is  due  to  fineness  of 
grain,  and  the  flotation  of  these  grains  gives  the  mass  its  mo- 
bility). This  fine  material,  vibrating  back  and  forth  through 
the  varying  capillary  currents,  will  materially  aid  in  keeping 
the  coarser  material  afloat. 

If  we  could  greatly  magnify  a  drop  of  slip  we  would  ob- 
serve a  condition  something  like  sketch  No.  1.  On  the  surface 
we  would  have  the  wavy  conditions  as  shown.  The  grains  of 
clay,  being  heavier  than  water,  do  not  float,  but  are  held  sus- 
pended at  the  surface  by  adhesion  of  the  water  to  the  grain 


DRYING    CLAY    WARES 


surfaces  and  surface  tension  of  the  water.  Should  two  parti- 
cles approach  each  other,  as  at  "A,"  a  capillary  current  is  set 
up  between  them,  the  tendency  of  which  is  to  carry  the  col- 
umn of  water  to  a  higher  elevation,  and  the  pressure  thus  de- 
veloped forces  the  particles  apart.  Thus  No.  1  particle  will  be 
driven  toward  No.  2.  Momentum  carries  it  beyond  the  point 
of  equilibrium  and  back  it  goes  toward  No.  3. 

If  we  let  the  slip  settle,  equilibrium  will  finally  be  reached, 


SUP6rnatant  water  we  still  find  our  clay 
Let  us  take  some  of  the  material  and  dry  it     In  the  fir«t 
N"?  ^  T™™  &S  Sh°Wn  in  No   * sketcl ^^    differ- 
sketch  N  TJ11  that  the  gminS  are  closer  to^er. 

^^^ 
are  drawn  together  by  surface  tension   they  exert 


DRYING    CLAY    WARES 


27 


a  compresive  force  on  the  mass  below,  squeezing  together  and 
forcing  the  water  outward. 

In  sketch  No.  4  we  are  approaching  the  final  stage.    The 
water  is  constantly  being  drawn  to  the  surface  and  evaporated 


Figure  5. 

and  the  water  in  the  pores  is  being  drained  just  as  if  a  wick 
had  been  inserted  into  each  pore.  The  end  is  reached  in 
sketch  No.  5,  where  all  the  pores  are  drained,  and  there  only 
remains  to  be  removed  the  water  clinging  to  the  surface  of 
the  grains. 


DRYING    CLAY    WARES 


This  is  removed  by  the  air  entering  the  pores  and  the 
evaporation  takes  place  from  the  surfaces  of  the  grains.  Even 
then  the  drying  is  not  complete.  The  clay  particles  cling 
tenaciously  to  the  water  and  the  last  traces  of  the  latter  can 
only  be  removed  by  temperatures  above  the  boiling  point,  but 
this  is  done  in  the  kiln  and  does  not  concern  us  in  the  drying. 

We  have  illustrated  the  grains  as  uniform  in  size  and 
shape,  but  in  reality  they  are  widely  varied,  as  illustrated  in 
sketch  No.  6.  As  compression  takes  place  the  angular  grains 
arrange  themselves  into  greater  compactness  and  the  finer 
grains  are  forced  into  the  interstices  between  the  larger 
grains. 

The  bond  or  strength  of  the  dried  ware  depends  upon  the 


Figure  6. 

surface  area  in  contact  and  the  forces  of  adhesion  and  co- 
hesion. A  lot  of  marbles,  or  shot,  or  rounded  grains  or  washed 
sand  will  have  little  or  no  bound.  There  is  too  little  surface 
in  contact.  Crush  the  marbles  and  mix  with  them  a  quantity 
of  infinitesimally  fine  materials  and  we  will  get  a  bond.  Add 
to  this  some  soluble  salt  which  as  evaporation  of  the  water 
proceeds  will  crystallize  and  interlace  the  mass  with  its  crys- 
tals and  we  get  a  still  stronger  bond. 

We  think  of  gummy  clays  which  are  so  difficult  to  dry  as 
being  chemically  different  from  other  clays,  but  the  difference 
is  largely  a  physical  one.  and  by  some  excellent  authorities 
is  considered  simply  a  difference  of  fineness  of  grain.  The 
fine  grains  necessarily  involve  small  pore  spaces  and  the 
water  travels  very  slowly  from  the  center  to  the  surface  of 


DRYING    CLAY    WARES 


19 


the  mass  and  in  consequence  is  troublesome  to  dry.  Others 
hold  that  these  gummy  clays  and  in  fact  plasticity  in  all 
clays  are  due  to  cells  or  sacs  enclosing  water  and  the  cells 
must  burst  to  allow  the  water  to  escape  and  also  to  provide 
passageway  to  the  surface  of  the  mass,  since  the  sacs  not 
only  hold  the  water  back,  but  pack  the  pore  spaces  through 
which  the  water  must  escape.  These  amorphous  sacs,  glue 
like  in  their  character  (colloidal),  as  drying  proceeds  coat 
the  larger  grains  and  cement  them  together.  When  colloids 
are  abundant  (gummy  clays)  drying  is  difficult,  but  the  dried 
ware  is  very  hard.  As  colloids  (colloidal  condition)  de- 
crease drying  is  safest  with  corresponding  falling  off  in  the 
strength  of  the  dried  ware. 


Figure  7. 

After  all,  these  colloidal  sacs  are  built  up  of  molecules 
and  held  together  by  cohesion,  and  so  far  as  we  are  con- 
cerned, fineness  of  grain,  in  connection  with  angularity, 
soluble  salts,  cohesive  and  adhesive  forces,  suffice  to  explain 
the  difference  in  the  bond  of  dry  clay  ware. 

Why  do  clay  ware  crack  in  drying?  Suppose  we  illustrated 
in  sketch  No.  7,  that  the  rate  of  evaporation  is  greater  at  the 
surface  than  the  rate  at  which  the  water  is  brought  to  the 
surface  by  capillarity.  The  air  will  follow  the  surface  of  the 
water  into  the  brick  as  at  "A."  The  surface  particles  can 
only  draw  together  as  the  entire  mass  shrinks,  and  the  mass 
can  only  shrink  as  the  water  is  driven  out.  Consequently 


DRYING    CLAY    WARES 


there  is  a  rupture  between  the  surface  grains.  The  crack 
started  at  "A"  will  go  deeper  and  deeper  into  the  ware  as 
drying  proceeds,  and  will  only  cease  when  sufficient  pores 
have  been  opened  by  the  crack  to  supply  the  rate  of  evapora- 
tion. 

The  cracks  in  such  instances  are  irregularly  hexagonal  in 
shape  (see  sketch  No.  8)  and  the  size  of  the  separate  masses 
depends  upon  the  difference  in  the  rate  of  evaporation  from 
the  surface  and  the  rate  of  the  progress  of  the  water  to  the 
surface. 

Many  wares,  such  as  bricks,  tiles,  fire  clay  blocks,  etc.,  of 
simple  rectangular  shape,  develop  straight  cracks  across  the 
narrow  face,  which  extend  into  and  across  the  ware.  Such 
cracks  are  due  to  the  load  being  in  excess  of  the  forces.  As 


Figure  8. 

noted  the  particles,  one  acting  on  another,  are  all  being  pulled 
toward  the  center  of  the  mass.  When  the  load  becomes 
greater  than  the  strength  of  the  chain,  a  break  occurs,  and 
the  load  is  divided  into  two  sections,  or  three,  or  a  dozen,  as 
the  case  may  be. 

Time  is  an  important  factor.  If  we  tie  a  string  to  a  load, 
and  give  it  a  quick  jerk,  we  break  the  string,  but  if  we  pull 
gradually  we  may  move  the  load.  Similarly  in  drying  if  we 
give  the  acting  forces  time,  we  can  safely  dry  any  ware 

Irregular  shaped  ware  develops  cracks  in  the  weakest 
Points,  due  to  the  shrinkage  forces  acting  in  opposite  direc- 
tions. For  example,  sketch  9  shows  a  shape  that  would  be 
ifficult  to  dry.  The  forces  pulling  the  two  halves  together 
are  greatly  reduced  in  the  neck  with  a  heavy  load  on  each 


DRYING    CLAY    WARES 


side  to  be  moved  toward  the  center.  An  "L"  shaped  piece 
will  crack  in  the  angle  because  each  leg  is  pulling  toward 
its  center  and  away  from  the  angle. 

Many  cracks  which  develop  in  drying  are  due  to  faulty 
structure.  Lamination,  for  instance  (sketch  10),  which  has 
been  previously  discussed,  is  really  a  core  within  a  shell.  The 
structural  fault  between  the  two  causes  a  break  in  the  flow 
of  water  from  center  to  surface  and  cracking  occurs. 


Figure  10. 

Hollow  ware  often  develops  straight  cracks  the  length  of 
the  ware,  due  to  weak  structure.  The  cracks  may  occur  in 
the  corner  where  we  have  the  effect  of  the  "L"  shaped  ware 
cited  above,  and  in  such  cases  the  trouble  is  not  necessarily 
due  to  faulty  structure. 

When,  however,  the  longitudinal  cracks  are  in  the  sides 
of  rectangular  ware,  and  also  in  circular  tile,  the  trouble  is 


DRYING    CLAY    WARES 


generally  due  to  structure.  The  core  bridge  splits  the  ware 
and  while  the  split  is  closed  up  in  coming  from  the  die,  yet 
not  to  have  the  strength  of  the  other  parts  of  the  ware.  It  is 
the  same  condition  as  sketch  9— the  weak  neck  is  present  but 
not  visible  in  the  ware  as  it  comes  from  the  die. 

This  structural  weakness  has  been  experienced  in  changing 
frqm  one  product  to  another. 

Many  manufacturers  who  have  installed  new  dryers  dis- 
credit the  dryer  because  it  will  not  dry  in  the  time  specified. 
When  told  to  increase  the  heat  or  circulation  they  say  it 
cracks  the  ware.  What  they  wanted  was  a  dryer  that  would 
do  the  work  in  a  specified  time  without  cracking  the  ware. 
The  trouble  is  in  the  clay,  in  the  shape  of  the  ware,  or  in 
some  structural  weakness  rather  than  in  the  dryer. 

We  have  discussed  methods  of  overcoming  structural  weak- 
ness, and  also  methods  of  improving  the  drying  qualities  of 
a  clay,  and  will  not  go  into  that  here. 

A  few  words  about  drying  mediums  other  than  air,  and  we 
will  close.  As  seen  from  the  table  previously  published,  the 
capacity  of  air  for  moisture  increases  very  rapidly  with  ad- 
vancing temperatures.  We  can  not  safely  increase  the  tem- 
perature in  a  tender  drying  clay,  because  we  soon  reach  a 
point  where  the  surface  evaporation  is  greater  than  the  in- 
ternal movement  of  the  water.  Suppose,  now,  that  we  intro- 
duce moisture  to  vapor  pressure  or  saturation.  We  may  then 
advance  the  temperature  to  any  degree  without  harm  to  the 
ware.  Many  tender  drying  clays  become  safe  drying  under 
this  treatment.  There  can  be  no  cracking,  since  no  drying 
can  take  place,  and  in  consequence  no  shrinkage  can  occur 
so  long  as  the  vapor  pressure  is  maintained.  Meanwhile  the 
heating  up  of  the  ware  sets  the  water  in  motion  and  pore 
spaces  are  cleared  for  the  subsequent  escape  of  the  water  to 
the  surface  when  drying  begins.  Moreover,  the  grains  of 
clay  are  being  softened  and  put  in  condition  to  adjust  them- 
selves more  readily  to  drying  strains.  We  have  only  to  ad- 
just the  degree  of  pressure  of  vapor  to  correspond  with  the 
rate  of  flow  of  water  to  the  surface  of  the  ware  to  insure  safe 
drying. 

This  humidity  treatment  to  insure  safe  drying  has  been 
extensively  used  in  terra  cotta  work  and  has  been  applied  to 
common  wares  with  gratifying  results  in  a  number  of  in- 
stances. 


DRYING    CLAY    WARES 


CHAPTER   V. 
Air  Drying. 

AIR  DRYING  of  clay  wares  is  both  ancient  and  modern. 
The  oldest  cities  of  the  world,  now  merely  mounds  in 
the  plain  they  once  adorned,  even  the  names  of  which 
are   questions  of  historical   dispute,   were   built  of  air-dried 
bricks,  and  the  .modern  city  of  New  York  is  likewise  largely 
built  of  the  same  product. 

By  air  drying  is  meant  drying  without  the  expenditure  of 
heat  except  such  heat  as  is  naturally  in  the  air,  or  such  as 
may  be  derived  from  radiation  from  burning  and  cooling  kilns. 
There  are  many  adaptations  of  air  drying. 

Open   Yard    Drying   of  Soft   Mud    Bricks. 

The  original  and  at  the  same  time  the  simplest,  is  to  lay 
the  ware  on  the  ground  exposed  to  wind,  sun  and  rain.  It  is 
limited  to  soft  mud  bricks,  and  there  are  many  clays  which 
will  not  stand  such  severe  drying  test.  It  seems  strange  that 
in  the  New  England  States,  New  York  and  New  Jersey,  where 
weather  conditions  are  least  favorable,  we  should  find  the 
largest  and  greatest  number  of  open  yards,  while  in  the  south 
and  west  where  conditions  are  extremely  favorable,  open  yards 
are  the  exception.  Perhaps  the  clay  has  much  to  do  with  it. 
Direct  exposure  of  a  green  piece  of  ware  to  the  sun's  rays  is 
a  very  trying  test,  and  open  yard  work  is  only  possible  where 
the  clays  will  come  through  the  ordeal  safely. 

The  Hudson  river  district  is  the  jnost  notable  instance  of 
open  yards.  For  many  years  open  yards  only  were  to  be  found 
in  this  district,  but  recently  the  artificial  dryers  are  coming 
into  use. 

The  arrangement  of  open  yards  is  much  alike.  See  Fig.  11. 
The  soft  mud  machines  are  widely  separatedj  and  each  has  its 
clay  mixing  rig,  thus  making  a  complete  plant  of  each  ma- 
chine. In  front  of  the  machine  is  a  broad,  practically  level 


34 


DRYING    CLAY    WARES 


space,  sufficiently  large  to  hold  the  daily  output  of  the  ma- 
chine with  space  for  hacking. 

Usually  the  work  begins  very  early  in  the  morning  and  the 
daily  task  is  on  the  yard  before  noon,  oftentimes  by  8  or  9 
o'clock  in  the  forenoon. 

The  molds  of  bricks  from  the  machine  are  placed  on  truck: 
and  run  to  the  yard,  and  are  there  dumped  on  the  ground,  thus 
covering  the  ground' with  bricks  laid  flat  and  spaced  the  thick- 
ness of  the  sides  and  divisions  of  the  mold.  The  early  start 


,  a,    0^ 


I \Mach<ne  I       \M&chinc  \       \J 

l~rV  U^-  U: 


J h 


Figure  11. 

is  necessary  in  order  to  gel  the  bricks  dried  before  night.  As 
soon  as  the  bricks  are  stiffened  so  they  will  hold  their  shape 
they  are  edged  up  with  a  wooden  tool  called  an  edger.  See 
Fig.  12.  The  divisions  of  the  edger  correspond  to  the  divisions 
in  the  mold.  The  edger  is  placed  over  six  bricks,  and  by  a 
dexterous  twist  the  six  bricks  are  turned  on  their  edges. 

The  bricks  on  edge  dry  until  later  in  the  day,  and  are  then 
hacked  up  in  long  hacks  across  the  yard  from  machine  to  kiln. 

The  advantage  of  open-yard  drying  is  in  initial  cost  of  in- 
stallation and  in  that  no  fuel  is  required.  The  disadvantages 


DRYING    CLAY    WARES 


35 


are,  short  operating  season  with  lessened  capacity  on  account 
of  bad  weather,  and  increased  labor  cost. 

An  open  yard  ware  is  limited  to  common  bricks,  the  de- 
mand for  which  is  light  during  the  winter  season,  especially 
in  the  north,  and  the  summer  operation  is  not  a  serious  handi- 
cap, since  the  initial  cost  of  the  plant  is  not  large  compared 
with  a  modern  plant  of  equal  capacity,  and  the  overhead  cost 
is  correspondingly  low.  The  yards  are  built  in  several  units 
and  a  yard  of  50,000  brick  capacity  may  have  machine  and 
drying  capacity  for  100,000  brick. 

During  the  busy  season  the  output  may  be  pushed  above 
the  normal  capacity.  Usually  one  or  more  .machines  are  idle 
all  the  time  in  order  to  keep  down  the  labor  cost.  For  in- 
stance, in  Fig.  11,  machines  1  and  2  will  be  operated  when 
setting  to  the  left  of  the  center  of  the  kiln  shed  and  machines 


/ 

JJ 

V                si 

Figure  12. 


2  and  3  when  setting  to  the  right.  This  gives  minimum  wheel- 
ing distance  from  the  hacks  to  the  kiln. 

It  can  be  shown  that  the  cost  of  putting  the  bricks  on  the 
yard,  edging  and  hacking,  on  the  basis  of  $2  labor  will  not  ex- 
ceed 25  cents  per  thousand.  It  is  also  evident  that  the  cost 
of  getting  the  bricks  from  hacks  to  kilns  in  view  of  the  mini- 
mum distance  ought  not  be  greater  than  from  a  mechanical 
dryer  however  centrally  located  it  may  be. 

The  fuel  cost  in  an  artificial  dryer  may  exceed  the  total 
cost  of  labor  incident  to  drying  on  an  open  yard  and  in  the 
tmost  economical  operation  will  equal  one-half  the  open  yard 
labor  cost. 

An  artificial  dryer,  however,  does  not  eliminate  labor  cost 
entirely,  but  does  reduce  it.  We  are  safe  in  saying  that  the 
labor  cost  will  not  be  less  than  one-half  the  cost  on  an  open 
yard.  The  labor  putting  the  bricks  into  the  dryer,  taking  them 
out,  handling  pallets,  making  repairs,  etc.,  will  on  many  plants 


DRYING    CLAY    WARES 


equal  the  cost  on  open  yard.  Then  the  cost  of  equipment 
must  be  considered— tunnels,  fans,  heaters,  piping,  racks,  cars, 
tracks,  pallets— and  the  total  cost  of  artificial  drying  will  ex- 
ceed that  of  natural  drying. 

The  great  advantage  of  the  artificial  drying  is  that  the  man- 
ufacturer is  independent  of  weather  conditions  and  may  keep 
the  plant  in  continuous  operation  throughout  the  season,  or 
throughout  the  year  if  desired.  The  work  may  start  early  in 
the  season  and  the  early  product  catches  the  early  market 
when  left-over  stocks  are  exhausted,  and  the  late  fall  operation 
insures  a  stock  to  last  until  the  new  product  comes  in.  Thus 
the  artificial  drying  increases  the  yearly  output  and  reduces 
the  overhead  cost  per  thousand  to  more  than  offset  any  in- 
crease in  the  actual  cost  of  drying. 

It  will  generally  be  found  that  artificial  drying  costs  more 
than  natural  drying,  but  we  hold  that  in  the  majority  of  in- 
stances the  net  advantage  will  be  in  favor  of  the  artificial 
operation. 

Stiff  Mud  Bricks  in  Open  Yards. 

The  open  yard  drying  is  occasionally  used  for  stiff  mud 
bricks.  The  bricks  from  the  machine  are  placed  on  foot  pal- 
lets— 60  to  80  bricks — and  these  are  carried  to  the  drying  yard 
by  lifting  trucks  and  set  in  rows. 

They  are  covered  with  boards  as  occasion  jnay  require  to 
protect  them  from  the  direct  rays  of  the  sun  or  from  rain. 
When  dry  they  are  picked  up  by  the  lifting  trucks,  taken  to 
the  kiln  and  the  pallets  returned  to  the  machine. 

The  pallets  and  covers  are  the  only  equipment  over  the 
soft  mud  open  yard  and  there  is  an  advantage  in  that  the 
bricks  are  handled  in  larger  units.  All  that  has  been  said  rel- 
ative to  the  soft  mud  open  yard  applies  to  the  stiff  mud,  with 
the  exception  that  the  bricks  originate  at  one  place,  and  in 
drying  must  be  left  on  the  yard  several  days  to  one  week.  The 
ground  area  required  for  each  day  is  much  smaller  than  for 
soft  mud,  since  the  bricks  are  hacked  on  edge  eight  to  ten 
courses  high. 

Rack  and  Pallet  Yard. 

By  far  the  larger  number  of  summer  common  brick  yards 
use  the  rack  and  pallet  system.  This  is  necessary  because  the 
clay  will  not  stand  the  severe  test  of  the  open  yard  work. 

Long  sheds,  cqmmonly  called  "racks,"  are  built  as  shown 
in  Fig.  13. 

The  uprights  are  spaced  the  length  of  the  pallets,  each  pal- 
let holding  one  mold,  6  or  7  bricks.  To  the  uprights  are  nailed 


DRYING    CLAY    WARES 


ft un  Plan* 


DRYING    CLAY    WARES 


horizontal  cleats  spaced  vertically  the  width  of  the  bricks 
plus  the  thickness  of  the  pallet  plus  clearance.  The  length  of 
the  cleats  is  sufficient  to  hold  four  pallets-two  on  each  side 

The  racks  are  roofed  over  as  shown  in  Fig.  13,  and  the 
space  between  is  covered  by  hinged  doors  which  may  be 
opened  or  closed  as  .weather  conditions  demand.  The  racks 
are  built  ten  cleats  in  height  and  each  section  holds  forty 
pallets,  or  240  bricks.  The  pallets  are  about  35  inches  long, 
making  the  section  over  three  feet  on  centers.  A  rack  100 
feet  long  will  have  32  to  33  sections  and  will  hold  7,920  brick. 
Seven  racks  are  required  for  a  day's  run  of  50,000  brick,  or  42 
racks  for  the  week's  run. 


Figure  14. 

As  the  racks  are  spaced  7  feet  on  centers,  the  total  space 
required  is  29,400  square  feet — something  over  half  an  acre. 

The  above  described  racks  (Fig.  13)  are  extensively  used 
in  this  country  in  air  drying  soft  mud  bricks,  but  in  foreign 
countries  where  labor  is  cheaper  other  types  of  buildings  are 
used.  For  instance,  three  or  more  racks  may  be  put  under  one 
roof,  but  the  advantage  in  quick  and  uniform  drying  is  with 
the  single  rack. 

In  Germany  large  sheds  are  often  used  with  the  racks 
across  the  shed  (See  Figs.  14  and  15),  and  space  is  provided 
for  storing  the  dried  or  partially  dried  bricks  until  kiln  space 
is  available.  This  involves  extra  handling,  but  it  saves  space, 
and  what  is  more  important,  it  insures  a  supply  of  bricks  at 


DRYING    CLAY    WARES  39 

all  times  for  the  kilns — a  consideration  not  to  be  overlooked 
where  the  burning  is  done  in  continuous  kilns. 

In  German  practice  continuous  kilns  are  more  commonly 
used  for  caramon  bricks  than  in  this  country,  and  to  keep  the 
kiln  in  operation  it  is  necessary  that  there  be  a  supply  of  dry 
bricks,  regardless  of  weather  conditions,  which  requires  not 
only  excessive  drying  space,  but  space  must  be  provided  to 
store  dry  bricks  in  order  to  take  advantage  of  good  drying 
weather  and  tide  over  periods  of  bad  weather.  The  atmos- 
pheric conditions  in  Germany  are  also  less  favorable  for  air 
drying  than  in  this  country,  which  accounts  for  the  more  per- 
manent structures  used  in  that  country. 


Figure  15. 

The  German  bricks  are  nearly  five  inches  wide  and  ten 
inches  long  (2\/2  centimeters  by  25  centimeters).  Their  racks 
are  usually  built  to  hold  only  two  pallets,  one  on  each  side, 
while  ours  hold  four.  They  also  use  higher  racks  than  we, 
which  requires  that  the  workmen  stand  on  benches  in  filling 
the  upper  racks. 

In  order  to  economize  space,  the  German  sheds  are  often 
built  two  stories  high  which  is  never  done  in  this  country. 

Because  of  the  unfavorable  atmospheric  conditions,  and  in 
order  to  protect  the  ware  from  early  and  late  frosts,  thereby 
getting  a  longer  drying  season,  the  Germans  have  largely 
adopted  the  method  of  constructing  the  drying  sheds  over  the 
continuous  kilns,  thus  taking  advantage  of  the  radiated  and 


40  DRYING    CLAY    WTARES 

waste  heat  from  the  continuous  kilns.  This  will  be  fully  de- 
scribed in  a  following  article.  We  are  beginning  to  adopt  this 
method  in  this  country,  and  it  is  likely  that  with  the  adoption 
of  the  continuous  kiln  for  conynon  bricks  this  method  of  dry- 
ing will  largely  replace  the  open  yard  or  yard  rack  work.  Now 


Figure  16. 


we  used   scove  kilns  which  are  independent  of  drying  con- 

k  n  to  st     d^  bUUd  S6Veral  arCheS  in  °ne  day  or  allow  the 
m?y  requirne    "  ^^  P6ri°d'  as  the  maki^  -d  drying 


Returning  to  the  subject  of  racks-at  the  machine   the 


DRYING    CLAY    WARES 


41 


bricks  are  dumped  on  flat  pallets,  usually  on  a  turn- 
table, and  the  loaded  pallets  are  trucked  to  the  racks  and 
placed  on  the  cleats.  As  soon  as  the  bricks  are  hardened  so 
they  will  stand  handling,  they  are  edged  up  by  hand.  When 
the  bricks  are  dry  they  are  removed  from  the  pallets  to  bar- 
rows and  wheeled  to  the  kiln. 

The  time  of  drying  varies  from  two  to  three  days  up  to 
ten  days,  depending  upon  the  weather.  • 

Compared  with  the  open  yard  work,  there  is  a  slight  ad- 
vantage in  labor  in  favor  of  it  over  the  rack  system,  -but  this 
is  more  than  offset  by  the  less  loss  in  the  rack  on  account  of 


Figure  18. 

the  protection  from  inclement  weather.  The  latest  develop- 
ment in  the  rack  system  is  to  use  rope  conveyors  from  the 
.machine  along  the  front  of  the  racks  with  cross  conveyors 
down  each  aisle. 

Air   Drying   Stiff   Mud    Bricks. 

In  place  of  the  movable  covers  for  stiff  mud  bricks  on  open 
yards  we  occasionally  find  a  shed  with  a  swinging  roof  as 
shown  in  Fig.  16.  Such  a  shed  economizes  the  labor  in  han- 
dling the  covers. 

A  more  permanent  structure,  however,  is  usually  built  for 
stiff  mud  bricks.  The  shed  structure,  a  section  of  which  is 
shown  in  Fig.  17  and  Fig.  18,  covers  all  the  drying  ground  and 


DRYING    CLAY    WARES 


is  a  permanent  structure.  Runners  about  four  inches  to  six 
inches  high  are  placed  through  the  shed  spaced  to  receive  the 
pallets  and  to  serve  as  guides  for  the  two-wheeled  lifting 
trucks.  Eighty  to  one  hundred  bricks  are  placed  on  pallets 
at  the  machine  and  run  to  the  drying  shed,  and  when  dry  are 
removed  to  the  kiln.  The  same  method  is  followed  in  handling 
larger  units  with  lifting  cars,  only  the  pallet  supports  (stanch- 
ions) are  higher. 

In  the  latter  case  several  stanchions  on  turntables  are 
placed  along  the  take-off  belt.  The  empty  pallets  are  placed 
on  the  stanchions  and  as  soon  as  one  side  is  filled  the  table 
is  turned  180  degrees,  bringing  the  empty  side  to  the  belt. 
When  the  pallet  is  full— 400  to  500  bricks— the  lifting  car  is 


Figure  19. 

run  on  the  turntable  under  the  load  and  the  latter  is  picked  up 
and  carried  to  the  dryer,  and  thence,  when  dry,  to  the  kilns. 

The  chief  advantage  is  that  the  bricks  are  handled  in  units 
of  400  to  500  bricks,  and  we  have  the  economy  of  handling 
which  comes  from  these  larger  units. 

Occasionally  steam  pipes  are  installed  under  the  racks  to 
hasten  the  drying  but  more  particularly  to  protect  the  green 
bricks  fro,m  frost  in  early  spring  and  late  fall  months. 

It  is  not  uncommon  practice  to  keep  off  frosts  by  the  use 
of  smudge  fires,  and  the  large  sheds  used  in  Germany  are 
better  adapted  to  this  than  our  open  racks.  It  is  well  known 
that  pure  air  absorbs  little  or  no  radiated  heat  either  from 
the  sun  or  from  the  earth.  Air  is  heated  by  conduction  in 


DRYING    CLAY    WARES  43 

contact  with  the  earth  and  is  distributed  by  convection  (circu- 
lation or  mass  movement).  Particles  of  dust  in  the  air,  how- 
ever, can  be  heated  by  radiation  and  in  turn  give  up  their 
heat  to  the  air  by  conduction. 

By  surrounding  the  bricks  with  smoke  particles,  if  we  may 
so  express  it,  we  collect  the  radiant  heat  from  the  earth  and 
from  objects  on  the  earth  and  retain  it  as  a  blanket  protec- 
ing  the  bricks. 

Drain  Tile  Sheds. 

Sheds  for  drain  tile  for  natural  drying  are  built  more  sub- 
stantially than  brick  racks.  A  common  type  is  a  shed  about 
fourteen  feet  wide  with  racks.  The  racks  are  on  either  side 
with  a  passage  way  through  the  center  of  the  shed. 

The  pallets  to  hold  the  tile  are  four  to  five  feet  long  and 
are  placed  in  the  racks,  just  as  brick  pallets  are  racked.  The 
tile,  however,  are  trucked  to  the  shed  and  placed  on  the  pal- 
lets in  place.  As  each  pallet  is  filled,  the  next  pallet  is  placed, 
and  so  on  until  the  rack  is  full.  Fig.  19  shows  a  section  of  a 
drain  tile  shed.  ' 

There  are  many  modifications  of  tile  sheds — often  same  old 
building  being  adapted. 

Very  little  can  be  said  in  favor  of  natural  drying  for  drain 
tile.  The  best  market  is  in  the  late  fall,  winter  and  early 
spring  and  a  tile  plant  equipped  for  natural  drying  only  is 
badly  handicapped.  A  good  combination  for  a  small  yard  is 
open  yard  drying  for  bricks  during  the  summer,  when  the  de- 
mand for  bricks  is  greatest,  and  an  artificial  dryer  for  tile 
during  the  winter.  Many  tile  makers  have  found  the  need  of 
artificial  drying  and  have  added  to  the  sheds  some  system  of 
heating.  Sometimes  steam  pipes  are  placed  along  the  floor 
under  the  racks,  and  we  have  seen  tunnels  or  flues  of  sewer 
pipe  built  under  the  racks  and  heated  from  a  furnace  at  one 
end  of  each  tunnel,  with  draft  stack  at  the  other  end,  or,  if 
the  shed  be  long,  the  stack  is  placed  midway,  with  a  furnace 
at  each  end  of  the  tunnel. 

Such  methods  of  heating  are  makeshifts  and  very  crude. 
They  art  not  to  be  recommended  in  a  new  construction  and 
will  not  be  considered  in  connection  with  artificial  drying. 


44  DRYING    CLAY    WARES 


CHAPTER  VI. 

The  Drying  Above  Continuous  Kilns. 

y  N  DRYING  ABOVE  a  continuous  kiln,  as  it  is  done  quite 
generally  in  Germany,  and  to  a   limited   extent  in  this 
1     country,  there  are  two  main  problems  involved,  viz:   the 
drying  and  the  handling  of  the  ware. 

Prior  to  the  introduction  of  the  continuous  kiln,  drying 
was  almost  exclusively  done  in  sheds  or  in  the  open  air  by 
sun  and  wind,  but  following  the  successful  operation  of  the 
Hoffman  kiln  it  was  quite  natural  and  logical  to  go  a  step 
farther  in  the  economical  performance  of  this  kiln  by  utilizing 
its  radiated  heat  to  assist  the  natural  air  in  drying  the  ware. 

The  first  dryers  thus  constructed  did  not  use  any  other 
heat  source,  but  merely  took  advantage  of  the  heat  radiating 
from  the  kiln  and  the  bricks  were  stacked  all  around  and 
above  it. 

Such  dryers,  one,  two  and  even  three  stories  high  can 
still  be  found  in  Germany  on  a  number  of  yards. 

Theoretically,  as  will  be  seen  later,  there  is  nearly  enough 
heat  from  a  continuous  kiln  to  dry  the  ware  to  be  burned  in 
the  kiln,  but  practically  so  much  of  it  is  lost  in  the  applica- 
tion, in  sufficiently  heating  and  maintaining  the  temperature 
of  the  air  to  carry  the  moisture  taken  up,  in  radiation  loss 
from  the  buildings,  in  heating  the  ware  itself,  etc.,  that  it  falls 
far  short  of  the  requirement. 

It  has  been  shown  that  the  radiation  loss  from  a  continu- 
ous kiln  is  32  per  cent,  of  the  fuel  consumed  in  the  burning. 
If  we  assume  that  200  pounds  of  coal  per  thousand  bricks  are 
used  in  the  burning,  then  we  will  have  the  value  of  64  pounds 
in  radiated  heat.  On  the  basis  of  12,000  B.  T.  U.  per  pound 
of  coal,  we  have  768,000  B.  T.  U.  in  radiated  heat.  If  the 
bricks  (American  size)  contain  one  pound  of  water  each, 


DRYING    CLAY    WARES 


which  requires  970  B.  T.  U.  to  evaporate,  then  the  evapora- 
tion of  the  water  in  1,000  bricks  will  consume  970,000  B.  T.  U. 
The  available  heat,  if  all  of  it  could  be  used  in  drying  alone, 
will  suffice  to  dry  800  bricks  out  of  each  1,000  to  be  burned, 
but  we  undoubtedly  lose  more  than  half  of  the  heat  from  the 
kiln.  As  the  fuel  required  for  burning  increases,  we  have 
corresponding  increase  in  the  available  radiated  heat,  and  it 
may  be  possible  in  factories  with  high  fuel  consumption  and 
properly  constructed  buildings,  together  with  the  best  appli- 
cation of  the  heat,  to  dry  the  ware  during  the  summer  season 
without  auxiliary  heat  supply. 

It  has  been  demonstrated  that  in  the  majority  of  instances 


Figure  20. 

the  radiated  heat  by  no  means  suffices  to  dry  as  much  ware 
as  the  continuous  kiln  can  burn,  and  it  has  been  found  that 
factories  depending  entirely  upon  radiated  heat  cannot  op- 
erate during  the  winter.  It  is  necessary,  therefore,  to  have 
additional  drying  facilities,  either  in  outside  sheds  or  by 
means  of  other  sources  of  heat  than  that  radiated  from  the 
kiln. 

Fig.  20  shows  a  factory  with  outside  sheds  contiguous  to 
the  kiln.  Even  with  such  outside  sheds  for  additional  drying, 
on  many  yards  the  bricks  are  taken  from  the  racks  as  soon 
as  practicable  and  are  racked  on  the  ground  around  the  kiln, 
and  near  to  it,  as  seen  in  the  illustration,  where  they  may 


DRYING    CLAY    WARES 


become  fully  dry.  Thus  advantage  can  be  taken  of  favor- 
able drying  weather  and  a  stock  of  dry  bricks  accumulated 
to  keep  the  kiln  in  full  operation  during  bad  weather.  The 
photograph  is  of  a  German  brick  plant  of  average  size  mak- 
ing 12,000  to  15,000  (German  size)  bricks  per  day. 

The  outside  drying  racks  hold  about  130,000  bricks;  50,000 
bricks  are  placed  in  racks  around  the  kiln,  level-  with  the  top 
of  the  kiln,  and  get  the  heat  radiated  from  the  kiln  walls  and 
wickets,  besides  more  or  less  circulation  from  the  kiln  top; 
an  additional  50,000  are  placed  in  portable  racks,  forming 
tunnels  above  the  kiln,  which  will  be  described  later. 

The  temperature  above  the  kiln  during  the  summer  varies 
from  20  degrees  C.  to  40  degrees  C.  (68  degrees  F.  to  104  de- 
grees F.)  and  the  time  required  for  drying  is  from  five  to  ten 
days. 


Figure  21. 

In  this  plant  we  have  100,000  bricks  dried  by  heat  from 
the  kiln  and  130,000  bricks  dried  in  outside  sheds,  with  stor- 
age space  in  which  to  accumulate  dry  bricks  during  good 
weather,  yet  the  need  of  additional  drying  facilities  is  urgently 
felt. 

In  such  kiln  dryers  the  air  regulation  is  effected  by  open- 
ing or  closing  the  windows,  according  to  the  wind  direction 
It  can  readily  be  seen  that  the  drying,  besides  being  slow  is 
also  quite  irregular.  Sometimes  in  order  to  distribute  the 
heat  more  uniformly  a  sheet  metal  lining  with  openings  is 
Placed  below  the  drying  floors  and  the  results  have  often 
>een  bettered  by  doing  so.  Usually  the  floors  under  the  ware 
are  slotted  and  only  left  solid  in  the  aisles  to  compel  the  ris- 
ng  heat  to  pass  through  the  ware  before  escaping  through 
the  windows  or  louvres  of  the  monitor  above 

5  updraft  is  slow  and  where  condensation  occurs  under 


DRYING    CLAY    WARES  47 

the  roof  there  are  placed  a  few  steam  pipes  below  the  monitor 
to  heat  up  the  air,  prevent  condensation  and  increase  the 
speed  of  the  rising  air.  In  such  cases  the  main  updraft  will 
naturally  be  directed  towards  the  center  and  the  drying  of 
the  ware  in  the  side  racks  will  be  neglected.  One,  or  better, 
two  suction  fans  in  the  monitor  are  doubtless  more  effective. 
With  a  stronger  draft  throughout  which  the  fans  will  give, 
it  is  possible  to  force  the  air  to  the  side  racks  by  closing 
floor  openings  under  the  center  racks. 

In  Fig.  21  we  have  a  system  devised  and  built  by  Cohrs 
about  thirty  years  ago.  The  drying  racks  are  placed  on  both 
sides  level  with  the  top  of  the  kilns,  thus  relieving  the  kiln 
walls  of  the  dryer  load.  Vapor  stacks  are  placed  intermit- 
tently betw-een  the  racks,  being  connected  with  ducts  below 
the  ware.  The  heated  air,  coming  from  over  the  kiln,  is 
drawn  down  through  the  ware  and  escapes  through  bottom 
openings  into  the  under  ducts,  thence  to  the  stacks.  The  dry- 
ing space  is  confined  to  the  kiln  top  level,  and  hence,  if  pos- 
sible, the  ware  should  be  delivered  from  the  machine  on  this 
floor,  so  that,  after  drying,  the  ware  only  needs  to  be  lowered. 

Most  of  the  modern  kiln  dryers  make  use  of  the  exhaust 
steam  from  the  engine.  There  may  be  pipes  under  the  floors 
and  the  natural  updraft  system  be  used,  or  there  may  be  used 
steam  heating  coils  in  connection  with  a  fan  or  a  combina- 
tion of  a  heating  tank  with  a  hot  water  system. 

We  must  mention  the  fact,  however,  that  European  plants 
do  not  have  the  amount  of  exhaust  steam  available  in  our 
American  plants.  Their  engines  do  not  require  over  10 
pounds  of  steam  per  h.p.  hour — from  one-half  to  one-fourth  of 
the  consumption  in  this  country — and  hence  the  heat  derived 
from  this  source  is  comparatively  small. 

Other  kiln  dryers  make  use  of  the  heat  from  the  cooling 
chambers  by  means  of  a  small  fan.  Many  plants  use  both 
exhaust  steam  and  the  heat  from  the  cooling  chambers.  Any 
heat  taken  from  the  kiln,  however,  is  not  gratis;  it  must  be 
replaced  in  some  way  and  more  fuel  in  the  burning  is  the 
result. 

It  furthermore  is  a  fact  that  the  so-called  radiated  heat  is 
not  always  to  be  considered  as  being  gratis.  Whenever  an 
artificial  updraft  above  the  kiln  is  created  by  fans  or  any 
other  means  and  the  air  taken  from  the  top  or  surroundings 
of  the  kiln,  there  is  bound  to  be  a  fall  of  temperature  of  the 
kiln  walls  and  thus  indirectly  of  the  chambers  and  the  heat 
taken  must  be  replaced  by  more  fuel  in  the  kiln.  The  actual 


48 


DRYING    CLAY    WARES 


Figure  22 


DRYING    CLAY    WARES 


49 


amount  of  radiated  heat,  therefore,  which  has  no  value  in  the 
kiln  operation  and  which  is  an  absolute  loss  except  as  It  may 
be  recovered  in  drying,  is  much  less  than  is  generally  claimed, 
but  at  the  same  time  it  may  be  turned  into  profits  provided 
its  value  is  not  more  than  offset  by  increased  labor  cost. 

As  regards  the  handling  of  the  ware  there  are  several 
systems  in  vogue.  Frequently  a  tray  elevator  is  used  for 
getting  the  ware  from  the  machine  below  to  the  upper  drying 
floors.  This  elevator  is  located  close  to  the  cutter,  as  we 
can  see  from  Fig.  22.  One  man  takes  off  two  or  three  bricks 
at  a  time  and  places  them  upon  the  tray  at  hand.  The  loaded 
trays  go  up  and  on  their  downward  movement,  after  passing 
the  head  sprockets,  are  unloaded.  It  is  essential  that  empty- 


Figure  23. 

ing  and  filling  the  racks  keep  pace  with  the  progress  of  the 
fires  in  the  kiln  as  far  as  possible  in  order  that  the  bricks 
in  greatest  need  of  heat  shall  be  over  the  hottest  part  of  the 
kiln.  Where  the  drying  room  has  two  floors  a  -man  on  the 
upper  floor  unloads  alternate  trays,  leaving  the  intermediate 
trays  for  the  man  on  the  lower  floor,  thus  the  racks  in  both 
floors  are  equally  filled  at  the  same  time. 

There  are  several  ways  of  conveying  the  ware  about  the 
drying  floor,  the  most  common  perhaps  being  the  car  system, 
of  which  there  are  several  in  use.  We  have  a  series  of  il- 
lustrations before  us  of  one  such  system,  not  necessarily  un- 
derstood to  be  the  best.  In  Fig.  23  we  see  the  upper  part  of 
the  tray  elevator.  The  man  in  front  of  it  is  taking  off  the 


50  DRYING    CLAY    WARES 

bricks  and  placing  them  in  the  frame  to  the  left,  which  is 
standing  on  two  wooden  blocks,  or  footings,  on  top  of  a  turn- 
table. The  frame  consists  of  seven  shelves,  each  made  up 
of  four  narrow  strips  of  wood,  so  that  a  brick  is  always  rest- 
ing on  two  strips.  After  one  side  is  filled  the  turntable  with 
frame  is  swung  around  180  degrees  and  the  other  side  is 
filled. 

As  soon  as  this  is  done  a  man  pushes  a  car  of  special 
design  under  the  frame  between  the  footings  and,  by  lifting 
a  lever,  catches  the  frame  under  the  upper  shelf  on  two  pro- 
jecting arms.  In  Fig.  24  we  see  the  frame  just  being  taken 
off,  while  in  Fig.  25  we  have  the  car  with  its  leverage  to  the 


Figure  24. 

rear  and  the  two  projecting  arms  to  the  front,  ready  to  be 
pushed  under  the  frame. 

The  loaded  car  is  moved  to  the  transfer  car  and  with  it 

is  pushed  along  until  opposite  the  space  being  filled.     The  car 

run  off  the  transfer,  pushed  into  the  space  and,  by  lower- 

ing the  lever,  the  frame  is  set  down  on  two  projecting  floor 

earns.     In  Fig.   26  we   see   the  man  setting  down  the  last 

frame  and  thus  filling  the  space  or  so-called  tunnel 


rowi  ey  are  loaded  °n  wheelbar- 

rows  and  lowered  on  a  double  gravity  elevator  to  the  ground 

main     "  h    T°  *'  ChambeFS  °f  the  kto"     The  fra™*  re- 
main on  the  drying  floors,  and  when  empty  are  returned  to 


DRYING    CLAY    WARES 


51 


Figure  25. 


52 


DRYING    CLAY    WARES 


the  turntable.  The  location  of  the  gravity  elevator  can  be 
seen  in  Fig.  20. 

Instead  of  this  system  the  frames  may  be  filled  in  front  of 
the  cutter  on  the  ground  floor,  then  the  loaded  frames  are  ele- 
vated to  the  drying  floor,  where  they  are  put  into  place  by 
lifting  cars  as  described,  or  by  similar  cars  of  a  better  design, 
and,  after  the  bricks  contained  therein  are  dry,  are  taken 
down  on  a  gravity  elevator.  A  better  plan,  which  will  be  de- 
scribed later,  eliminates  the  portable  frames,  and  the  pallets 
only  are  handled  by  the  lifting  cars. 

Again,  another  way  is  to  use  a  combination  of  tray  ele- 
vator and  tray  conveyor  with  the  elimination  of  cars  entirely. 


Figure  26. 


There  are  numerous  arrangements  adapted  to  special  con- 
ditions and  to  take  advantage  of  different  methods  of  handling 
the  bricks  and  distributing  the  heat. 

Fig.  27  shows  an  underground  continuous  kiln  with  dryer 
on  the  ground  level  on  either  side.  The  bricks  are  partially 
dried  in  the  racks  around  the  kiln  and  are  then  stacked  on 
the  cooling  burned  bricks  for  the  final  drying.  They  do  not 
need  to  be  elevated;  they  are  delivered  by  the  machine  to  the 
racks  on  the  ground  floor,  thence  removed  to  the  kiln,  and 
later  lowered  into  the  kiln  for  burning.  This  scheme  requires 
a  second  handling  in  the  drying.  In  Fig.  28  we  have  the  same 
kiln  with  a  dryer  building  above. 


DRYING    CLAY    WARES 


Fig.  29  illustrates  a  continuous  kiln  with  three  drying 
floors  above.  It  is  evident  that  in  this  case  the  load  of  the 
dryer  upon  the  kiln  walls  and  piers  is  rather  excessive  and 
first  quality  masonry  and  good  foundation  work  are  essential. 

Pig.  30  illustrates  an  arrangement  first  designed  by  Schaff 
and  later  taken  up  and  now  built  by  Rudolf  Witte  of  Osna- 
briick,  Germany,  for  drying  bricks  and  especially  roofing  tile. 
The  tiles  are  delivered  on  the  upper  ends  of  inclined  chutes 


Figure  27. 


BJ- 


Figure  28. 


and  gradually  slide  down  as  drying  proceeds  and  as  the  dried 
ware  is  removed  from  the  lower  ends  of  the  chutes  along  the 
outer  aisles.  Schaff  originally  used  the  natural  updraft  of 
the  hot  air  within  closed  chutes,  while  Witte  blows  in  hot  air 
from  the  side,  taken  from  the  cooling  chamber  or  a  heater. 
The  original  idea  may  not  be  more  effective,  but  it  certainly 
is  more  interesting  on  account  of  its  scheme  by  which  the 
drying  medium  (the  air  going  up)  and  the  conveying  feature 


54 


DRYING    CLAY    WARES 


(the  ware  coming  down)  carries  out  a  logical  idea  which  has 

been  so  successfully  applied  in  our  progressive  tunnel  dryers. 

In  Fig.  31  we  have  the  bricks  in  racks  around  the  kiln  at 


Figure  29. 


Figure  30. 

the  kiln  top  level  and  similar  to  the  Cohrs  system  shown  in 
Fig.  21,  which  relieves  the  kiln  of  the  excessive  weight  in  other 
systems  where  the  bricks  are  above  the  kiln.  This  system 
only  uses  radiated  heat  from  the  side  walls  of  the  kiln  and 


DRYING    CLAY    WARES 


55 


that  escaping  from  the  wickets.  Under  the  racks  are  placed 
steam  pipes  enclosed  in  a  box,  with  admission  for  air  on  the 
sides  next  to  the  kiln,  and  outlets  into  racks  on  the  opposite 


Figure  32. 

upper  side.  The  principal  source  of  heat  in  this  instance  is 
from  the  steam  pipes,  and  not  from  the  kiln.  The  radiated 
heat  from  the  top  of  the  kiln  is  used  to  produce  draft  through 
the  monitor  of  the  building  and  is  not  available  for  drying. 


.    56  DRYING    CLAY    WARES 

Fig.  32  gives  the  arrangement  as  built  by  F.  L.  Smith  & 
Co.  of  Copenhagen  and  Berlin.  There  are  certain  features  in 
the  drying  as  well  as  in  the  handling  worth  describing,  and 
we  will  follow  the  ware  as  it  leaves  the  machine.  The  bricks 
are  cut  from  the  end  of  the  bar  of  clay,  three  at  a  time,  with- 
out waste,  by  a  hand  cutter  operated  by  one  man  and  a  ca- 
pacity of  40,000  to  50,000  German  size  bricks  is  attained  per 

day a  remarkable  output  for  a  hand  cutter  operated  by  one 

man.  Such  a  cutter  is  shown  in  Fig.  25.  The  bricks  are  taken 
off  by  another  man  and  placed  on  a  pallet  which  is  level  with 
the  cutter.  There  are  ten  such  pallets,  each  of  which  will 
hold  fifteen  bricks,  resting  loosely  on  the  frame  of  an  elevator 
and  the  filling  starts  with  the  lowest  pallet.  As  soon  as  the 
bottom  pallet  is  filled  the  elevator  drops  the  height  of  one 
pallet,  bringing  the  second  pallet  level  with  the  cutter  and 
thus  repeating  until  the  frame  is  full.  A  pit  receives  the  ele- 
vator as  it  descends.  A  woman  on  the  opposite  side  of  the 
elevator  frame  spaces  the  bricks  on  the  pallets. 

After  the  frame  is  loaded  the  woman  shifts  the  lever  of 
the  coupling  and  the  elevator  rises  to  the  floor  where  the 
bricks  are  to  be  dried.  Two  elevator  frames  at  right  angles 
to  each  other  forming  a  letter  V,  the  angle  of  which  encloses 
the  cutter,  are  used.  Thus  the  take-off  is  at  equal  distance 
from  both  elevator  frames  and  has  to  make  only  a  quarter 
turn  to  place  the  bricks  on  either  frame. 

When  the  first  frame  has  reached  the  upper  drying  floor 
the  elevator  is  stopped  automatically  and  a  man  with  a  spe- 
cial car  takes  off  the  row  of  pallets  from  the  elevator  by 
pushing  the  car  with  its  ten  sets  of  projecting  arms  into  it 
and  raising  the  pallets  from  the  frame  supports  by  the  move- 
ment of  a  lever  on  the  car.  After  the  car  is  loaded  and  pulled 
back  the  elevator  frame  is  filled  with  (ten)  empty  pallets  and 
lowered  to  the  first  position  at  the  cutter.  The  drying  sec- 
tions or  tunnels  are  provided  with  projections  to  receive  the 
pallets  corresponding  to  the  projections  on  the  elevator  pallet 
frame,  and  the  loaded  pallets  are  placed  in  position  by  a  single 
movement  of  the  car  lifting  lever.  One  man  on  the  dryer  can 
easily  handle  30,000  bricks. 

Turning  again  to  our  illustration,  Fig.  32,  we  see  the  dry- 
ing floor  located  to  one  side  of  the  kiln  to  avoid  any  load  upon 
the  kiln  walls.  Outside  air  is  coming  in  from  the  opposite 
side,  passes  over  the  top  of  the  kiln  and  becomes  heated  by 
contact  with  the  kiln  top  and  by  commingling  with  hot  air 


DRYING    CLAY    WARES 


57 


58 


DRYING    CLAY    WARES 


DRYING    CLAY    WARES  59 

rising  from  the  kiln.  An  air-tight  ceiling  prevents  the  heat 
from  escaping  into  and  through  the  roof,  thus  forcing  it  to 
enter  the  dryer  on  the  side  facing  the  kiln.  The  heat,  after 
having  passed  through  the  ware,  enters  a  vapor  stack  pro- 
vided with  regulating  dampers  and  escapes  to  the  open  air. 

The  speed  of  the  air  in  the  dryer  Is  about  60  meters  (197 
feet)  per  minute.  The  exhaust  steam  from  the  engine  is  also 
used  for  drying  purposes.  The  steam  is  forced  into  a  well 
insulated  hot  water  tank,  which  supplies  ribbed  pipes  under 
the  dryer.  The  circulating  water  will  gradually  cool  off  and 
return  to  the  tank  by  its  own  weight  through  separate  piping. 

The  drying  chambers  are  made  as  small  as  possible  to 
utilize  all  the  space  and  to  avoid  losses  of  heat. 

A  test  of  such  a  dryer  some  years  ago  during  the  month  of 
November  gave  the  following  data: 

The  temperature  of  the  outside  air  was  11  degree  C.  (52 
degrees  F.)  and  its  degree  of  saturation  90  per  cent.  The 
temperature  of  the  air  entering  the  dryer  was  found  to  be  15 
degrees  C.  (59  degrees  F.)  and  its  degree  of  saturation  65  per 
cent.  The  temperature  rising  in  the  dryer  was  23  degrees  C. 
(73  degrees  F.).  The  temperature  in  the  vapor  stack  was  15 
degrees  C.  with  a  saturation  of  93  per  cent.  The  sectional 
area  of  the  stack  was  4.2  square  meters  (45.2  square  feet)  and 
the  speed  of  the  air  60  meters  (197  feet)  per  minute.  Each 
of  the  sixteen  chambers  contained  7,500  bricks  and  it  took  four 
days  for  drying.  At  59  degrees  F.  saturated  air  contains 
4.7")  grains  of  moisture  per  cubic  foot.  In  the  above  data  the 
air  entered  at  59  degrees  F.  65  per  cent,  saturated  and  came 
into  the  vapor  stack  at  59  degrees  F.  and  93  per  cent,  satu- 
rated. Sixty-five  per  cent,  and  93  per  cent,  of  4.75  give  us,  re- 
spectively, 3.08  and  4.41  grains,  and  the  difference,  1.33  grains, 
represents  the  amount  carried  out  by  each  cubic  foot  of  air. 
The  stack  being  45.2  square  feet  and  the  air  moving  at  the 
rate  of  197  feet  per  minute,  we  get  an  air  movement  of  8,904 
cubic  feet  per  minute,  carrying  11,842  grains  of  water.  In 
one  hour,  therefore,  about  123  pounds  of  water  are  carried 
out,  and  in  four  days  11,808  pounds,  which  fairly  represents 
the  water  in  7,500  (German  size)  bricks. 

Fig.  33  shows  the  same  system  for  two  continuous  kilns, 
for  which  it  works  out  especially  well  by  getting  the  dryer  in 
the  center. 

In  Fig.  34  we  have  a  system  of  drying  which  has  been  in 
successful  operation  in  this  country  for  a  number  of  years. 

The  bricks  are  set  on  long  trays,  which  are  fastened  to  a 


DRYING    CLAY    WARES 


iii 


^      /  '  £ 

fP^h-& 


& 


VK/ 

-k— — --• 


DRYING    CLAY    WARES  61 

continuous  double  chain  conveyor,  which  passes  over  and 
under  sprockets  the  full  length  of  the  continuous  kiln,  as 
shown  in  the  illustration,  and  returns  to  the  ground  floor, 
where  the  dry  bricks  are  removed  and  wheeled  to  the  kiln. 
The  conveyor  is  operated  by  rack  and  pinions  driven  from  a 
steam  cylinder,  as  seen  in  Fig.  35.  Alongside  one  wall  steam 
radiators  are  placed  and  the  whole  dryer  is  operated  at  pretty 
high  temperature,  higher  than  in  German  practice.  The 
bricks  are  handled  three  times  in  small  units — once  from  ma- 
chine to  tray,  once  from  tray  to  barrow  and  once  from  barrow 
to  setter. 

Prom  our  experience  with  cars  and  transfers  into  and  out 
of  dryer  tunnels  and  kilns,  we  believe  there  will  be  some 
economy  in  labor  in  the  above  system.  Our  continuous  kilns 
are  not  insulated  as  are  the  German  kilns,  and  we  will  have  a 
greater  value  in  radiated  heat,  but  still  far  from  sufficient  to 
dry  the  product  burned  in  the  kiln.  As  our  kilns  are  con- 
structed, operated  and  maintained  they  are  not  adapted  to 
carry  heavy  loads,  especially  loads  of  moving  machinery 
equipment,  which  must  be  kept  in  perfect  alignment. 

In  a  number  of  plants  drying  floors  have  been  established 
over  periodic  kilns,  but  the  consensus  of  opinion  is  that  such 
combination  of  kilns  and  drying  floors  is  not  satisfactory. 

There  have,  perhaps,  other  attempts  been  made  in  this 
country  to  dry  the  ware  above  kilns,  of  which  we  do  not 
know.  The  tendency  of  our  day  is  directed  towards  econ- 
omy in  fuel,  and  we  believe  that  the  methods  of  drying  above 
kilns  will  in  the  future  more  and  more  be  taken  into  serious 
consideration. 

In  conclusion  we  briefly  wish  to  recapitulate  the  advan- 
tages and  disadvantages  of  the  drying  methods  above  de- 
scribed: Advantages. 

Utilization  of  space. 

Compactness  of  arrangement.  (Eventually  the  whole  plant 
under  one  roof.) 

Economy  in  fuel.        Disadvantages. 

Elevating  and  lowering  of  the  ware. 

Load  upon  the  kiln  walls.  (The  outer  kiln  walls  should 
not  be  loaded  under  any  circumstances.) 

High  buildings. 

The  radiation  alone  not  sufficient  for  drying  and  in  conse- 
quence extensive  and  scattered  auxiliary  equipment. 


DRYING    CLAY    WARES 


CHAPTER   VII 
Artificial  Dryers. 

AMERICA  LEADS  in  the  development  of  artificial 
dryers  for  clay  ware,  and  her  advance  in  this  fea- 
ture of  clayworking  is  due  to  greater  need,  or  more 
properly,  to  greater  variety  of  needs.  A  need  leads  to  an  in- 
vention to  supply  it.  No  two  clays  are  exactly  alike  and  in 
the  variety  of  clays,  America  has  all  the  needs  in  the  world. 
The  alluviums  of  the  coasts  border  and  overlap  the  tertiary 
and  cretaceous  deposits,  which  in  turn,  extend  to  the  foot- 
hills of  shales,  adjacent  to  the  mountains  of  schists,  quartz- 
ites  and  hard  shales.  In  and  beyond  the  mountains  come 
the  rich  shales  and  fireclays  of  every  kind  laid  down  in 
Paleozoic  times,  and  these  overlap  the  calcareous  shales  of 
the  Devonian  age.  The  great  glacial  cap  stretches  in  a 
broad  belt  from  coast  to  coast,  nearly  three  thousand  miles. 
In  the  broad  valleys  are  the  deep  terrace  and  lake  deposits  of 
the  Champlain  period.  The  mountains  are  ribbed  and  seamed 
with  disintegrated  dikes  of  every  description.  In  the  Middle 
West  are  vast  deposits  of  white  cretaceous  clay,  overlapped 
by  tertiary  shales  the  drying  difficulties  of  which  are  beyond 
the  ingenuity  of  the  dryer  man.  Over  the  plains  are  found  the 
wind-tossed  beds  of  loess,  and  the  troublesome  joint  clay.  In 
the  South  are  the  washings  of  the  continent,  ancient  and 
modern,  shales  and  alluviums,  kaolins,  Fullers  earths,  fire 
clays,  bauxite. 

The  development  of  the  dryer  is  also  influenced  by  cli- 
matic conditions.  We  have  the  frozen  North  and  the  sunny 
South,  the  arid  plains  and  the  dripping  west  coast,  and  all 
kinds  of  climate  in  between. 

Fuel  also  must  be  considered.  In  a  smaller  country  the 
variation  in  cost  is  less  wide  than  in  our  broad  land  where  a 
difference  of  1,000  per  cent,  is  not  unusual.  In  the  coal  dis- 


DRYING    CLAY    WARES  63 

tricts,  where  coal  and  clay  often  come  from  the  same  pit, 
the  cost  of  fuel  is  of  slight  consideration,  while  in  the  dis- 
tant districts  it  must  have  every  consideration. 

High  priced  labor  must  be  reckoned  with,  and  every  effort 
is  made  to  develop  a  mechanical  operation. 

These  conditions  in  every  extreme  have  led  to  a  wide  de- 
velopment of  mechanical  dryers. 

All  dryers  can  be  placed  in  two  general  classes: 

1.  The  periodic  dryer,  in  which  the  ware  is  stationary 
and  the  heat  and  circulating  air  are  brought  to  it. 

2.  The  progressive  dryer,  in  which  the  ware  is  being  ad- 
vanced from  a  low  temperature,  usually  humid  zone,  to  a  high 
temperate  dry  zone. 

The  periodic  dryers  may  be  separated  into:  Floor 
dryers,  rack  dryers,  tunnel  or  compartment  dryers. 

The  progressive  dryers  are  limited  to  the  tunnel  or  com- 
partment type. 

A  further  classification  introduces  the  source  of  heat  as 
follows: 

Combustion — 

Introduced  direct. 
Radiation. 
Convection. 
Steam — 

Direct  radiation. 
Convection. 
Waste  Heat- 
Steam. 

Cooling  kilns. 
Burning  kilns. 

The  possible  variations,  the  combinations  and  the  modi- 
fications, to  adapt  dryers  to  all  kinds  of  ware  and  all  varieties 
of  clays,  gives  us  a  long  list  of  artificial  dryers. 

It  is  not  our  purpose  to  go  into  a  full  description  and 
discussion  of  all  the  modifications,  but  instead  we  will  take 
up  the  general  types  and  follow  with  some  of  the  best  known 
and  most  widely  used  modifications,  but  we  will  not  attempt 
to  follow  any  definite  classification.  In  fact,  the  division  is 
not  always  sharply  drawn.  It  is  but  a  step  from  hot  floors 
to  ordinary  drying  floors  in  one  direction  and  to  radiated 
tunnel  dryers  in  the  opposite  direction.  Periodical  dryers 
in  some  instances  approach  very  closely  to  the  progressive 


64  DRYING    CLAY    WARES 

type,  some  being  periodical  in  construction  and   progressive 
in  operation. 

General  Principles. 

Before  taking  up  the  individual  dryers,  we  wish  to  re- 
view briefly  the  general  principles  of  drying  even  though 
we  repeat  what  has  been  said  in  a  previous  chapter. 

1.  It  is  important  to  bear  in  mind   that  drying   cannot 
take  place  without  the  consumption  of  heat,  and  the   same 
amount    of   heat    is    required    in    every    instance    for    equal 
amounts  of  water  evaporated.     The  efficiency  of  a  dryer  de- 
pends upon  the  application  of  the  heat. 

2.  Air  has  nothing  whatever  to  do  with  drying  in  a  strict 
sense.     We   speak  of  the  volume  of  air  required   and  it.  is 
convenient  to  do  so.     It  is  true  that  we  use  air  in  drying, 
but  not  for  drying.    The  air  is  simply  a  vehicle  to  carry  away 
the  water  vapor,   or  more  properly  speaking,   it   is  used   to 
create  a  current,  or  a  draft,  to  sweep  away  the  moisture  vapor 
as  fast  as  it  is  formed.    As  a  matter  of  fact,  instead  of  air  ab- 
sorbing vapor,  the  latter,  as  fast  as  it  forms,  displaces  air 
by  its  pressure,  until  at  the  boiling  point  there  is  theoretic- 
ally no  air  present  in  a  vessel  containing  the  boiling  water. 
Some  types  of  dryers  have  small  air  inlets  and  equally  small 
outlets  with  no  other  force  moving  the  air  than  natural  draft. 
Other  types  use  fans  to  force  the  air  in  through  large  ducts 
in  which  one  may  walk  without  inconvenience,   and  at  the 
exhaust  end  of  the  dryer  is  another  fan  drawing  away  the 
air  and  moisture  through  another  large  duct.     These  dryers 
may  have  equal  drying  capacity  yet  there  is  no  comparison 
in  the  volumes  of  air  passing  through  them. 

3.  Other  things  being  equal,  the  greatest  economy  will 
come  with  the  shortest  connection  between  the  source  of  heat 
and  the  drying  ware. 

A  direct  coal  fired  combustion  dryer  will  give  greater  re- 
turn in  heat  than  a  dryer  in  which  the  fuel  is  used  to  gen- 
erate steam  a  hundred  or  more  feet  away  and  the  heat  value 
recovered  from  the  steam  in  or  adjacent  to  the  dryer,  on  the 
other  hand,  where  the  combustion  gases  cannot  be  used  di- 
rect, but  instead  the  heat  from  the  combustion  must  be  con- 
ducted through  walls  thence  by  radiation  to  the  ware,  the 
amount  of  heat  which  may  be  led  to  the  ware  may  be  less 
than  by  the  more  complicated  steam  operation. 

4.  Meteorologists  tell  us  that  air  cannot  be  heated  by 


DRYING    CLAY    WARES  65 

radiation,  but  becomes  heated  by  contact  with  hot  bodies 
(conduction),  and  the  movement  of  the  air  carries  the  heat 
from  place  to  place  (convection),  and  gives  it  up  by  con- 
duction, to  the  bodies  with  which  it  comes  in  contact.  They 
also  inform  us  that  heat  waves  radiating  from  a  hot  body  will 
raise  the  temperature  of  any  solid  body  with  which  they  come 
in  contact  without,  as  above  stated,  heating  the  intervening 
air.  The  Fery  pyrometer  is  perhaps  an  illustration  of  this. 
We  focus  it  on  a  glowing  body  in  the  center  of  a  kiln  and  it 
will  register  the  temperature  in  a  galvanometer,  but  instantly 
a  screen  is  intervened  the  temperature  drops  back,  which 
would  not  be  the  case  if  the  intervening  air  were  heated  by 
radiation  from  the  glowing  body.  We  sit  in  front  of  a  fire 
and  are  comfortable  so  long  as  a  screen  is  held  between  us 
and  the  fire  but  without  the  screen  our  face  will  burn.  It  is 
evident  that  the  burning  sensation  does  not  come  from  the 
air. 

5.  Dryers,  then,  must  be  constructed  to  get  the  air  in 
contact  with  the  heated  body  in  the  greatest  degree  and  then 
be  brought  in  contact  with  the  ware  to  be  dried,  or  if  we  are 
relying  upon  radiation  from  the  hot  floor  to  the  ware  there 
should  be  as  little  movement  of  the  intervening  air  as  possible 
because  to  whatever  degree  it  comes  in  contact  with  the  hot 
body  and  ware  it  will  carry  away  heat  from  both,  but  on  the 
other  hand,  after  the  heat  has  driven  the  moisture  from  the 
ware  there  should  be  a  current  of  air  to  remove  the  vapor. 
Herein  lies  the  difference  in  the  quantity  of  air  required  for 
different  types  of  dryers.  In  one  type  the  air  is  heated  by 
contact  with  the  hot  body  and  is  then  brought  into  contact 
with  the  drying  ware.  In  the  other  type,  the  ware  is  heated 
and  the  water  evaporated  directly  by  the  heat  radiated  from 
the  hot  body  to  the  ware  and  it  is  only  necessary  to  maintain 
the  vapor  condition  and  sweep  it  away  either  by  its  own  ex- 
pansive force  or  by  a  current  of  air. 

Floor  Dryers — Hot  Floor. 

Clay  wares  were  originally  dried  on  the  ground  without 
cover.  Shelter  was  next  provided  which  raised  the  drying 
space  to  the  dignity  of  a  floor.  It  was  but  a  step  farther  to 
provide  some  method  of  heating  the  floor  and  this  led  to  the 
development  of  modern  hot  floors. 

The  hot  floor,  so  called,  was  first  used  in  the  manufac- 
ture of  fire  bricks  and  is  still  the  chief  drying  equipment  in 
such  factories.  It  consisted  of  a  floor  of  any  width  and  one 
hundred  feet  or  less  in  length.  At  one  end  was  a  firing  pit 
below  the  floor  level,  and  at  the  other  end  a  stack.  The  floor 
was  underlaid  by  a  series  of  parallel  flues  connected  directly 


DRYING    CLAY    WARES 


with  coal-fired  furnaces  in  the  firing  pit,  and  with  the  stack 
by  a  cross  head  flue. 

In  order  to  equalize  the  temperature  of  the  floor,  it  was 
made  thicker  near  the  furnaces,  gradually  decreasing  in 
thickness  toward  the  stack.  The  flues  were  spanned  with 
bricks,  then  the  thickness  was  built  up  with  rammed  ashes 
and  paved,  or  with  rammed  crushed  furnace  slag  without 
paving,  or  with  concrete  finished  to  a  smooth  surface.  Even 
at  best  it  was  impossible  to  get  the  temperature  uniform 
nor  was  it  considered  desirable  to  do  so,  because  some  wares 
required  slower  drying  than  others  and  a  suitable  temperature 
for  all  the  product  could  be  found  on  such  a  floor. 

The  size  of  the  floor  per  thousand  bricks  depends  upon 
the  time  required  to  dry,  and  varies  from  700  square  feet  to 
1,250  square  feet. 

In  the  hand  molding  process  the  bricks  in  the  molds  are 
carried  by  boys  from  the  molder  and  are  dumped  flat  on  the 
hot  floor  just  as  common  bricks  from  soft  mud  machines  are 
placed  on  the  open  air  drying  ground. 

The  bricks  are  left  on  the  floor  from  twelve  to  twenty- 
four  hours  or  until  dry  enough  to  repress.  If  they  show 
signs  of  becoming  too  dry  before  the  repressing  gang  gets 
them,  they  are  hacked  into  piles,  or  in  some  instances,  simply 
edged  up.  This  is  one  instance  where  the  drying  may  be 
retarded  by  edging  up  the  bricks,  where  usually  edging  is 
resorted  to  in  order  to  hasten  the  drying  process. 

Where  the  bricks  set  sufficiently  for  repressing  in  eight 
or  ten  hours  it  is  the  custom  for  the  molders  to  begin  work 
about  4  a.  m.  and  finish  their  task  by  noon,  while  the  re- 
pressing crew  will  begin  at  noon  or  as  early  as  the  drying 
will  permit,  and  continue  until  the  day's  output  is  complete. 
From  the  repress  the  bricks  are  again  placed  flat  on  the 
floor,  four  to  eight  high,  and  allowed  to  remain  until  com- 
pletely dry,  when  they  are  taken  up  and  wheeled  to  the  kilns. 
The  tunnel  dryer  is  being  introduced  in  fire  brick  manu- 
facture, as  an  adjunct  to  the  hot  floor,  and  in  fact  has  been 
in  use  on  a  few  yards  for  many  years.  It  is  used  for  the  re- 
pressed product  and  for  any  struck  bricks  which  do  not  have 
to  be  repressed. 

The  advantage  of  the  open  floor  is  that  the  bricks  can  be 
watched  and  repressed  when  in  the  proper  condition,  and  it 
is  retained  in  all  fire  brick  plants,  but  the  crude  direct  fired 
hot  floor,  expensive  in  fuel  consumption  and  at  best  not  very 
efficient,  has  been  replaced  by  the  modern  exhaust  and  live 
steam  heated  floor. 

Figs.  37,  38  and  39  show  a  section,  plan,  and  detail  of  a 


DRYING    CLAY    WARES 


Figure  37. 


Figure 


DRYING    CLAY    WARES 


modern  hot  floor.  The  floor  is  divided  into  sections  so  that 
each  can  be  heated  independently.  It  has  a  base  of  concrete 
upon  which  are  laid  4-inch  hard-burned  drain  tiles  or  elec- 
trical conduits  imbedded  in  and  covered  with  concrete,  which 
is  finished  to  a  smooth  surface,  with  the  tiles  as  near  to  the 
surface  as  practical. 

The  hot  floor  is  adapted  only  for  low  pressures  and 
size  and  number  of  tiles  under  the  floor  is  such  that  the 
steam  pressure  is  virtually  atmospheric  pressure.  Exhaust 
steam  is  used  during  the  day  and  low  pressure  live  steam  at 
night,  which  reaches  the  floor  at  a  gauge  pressure  less  than 
five  pounds.  The  main  header  is  connected  with  the  under 
tiles  by  %-inch  nipples  and  though  the  pressure  in  the 
header  be  five  pounds  the  drop  will  be  practically  to  atmos- 
pheric pressure  in  the  tiles,  being  merely  sufficiently  in  ex- 
cess to  carry  the  steam  to  the  exhaust  end  of  the  floor! 


Figure  39. 

The  tiles  have  a  grade  sufficient  to  carry  away  the  con- 
densation and  the  floor  surface  follows  this  grade. 

In  modern  plants  there  is  a  second  slatted  floor,  as  shown 
in  Fig.  37,  upon  which  are  molded  and  dried  the  large  and 
intricate  shapes  which  require  careful  drying  treatment.  The 
temperature  is  lower  in  the  second  floor  and  the  ware  cannot 
come  in  contact  with  the  hot  radiating  floor.  Moreover,  the 
air  is  partly  saturated  with  moisture  from  the  ware  on  the  hot 
floor,  and  in  consequence  the  progress  of  the  drying  on  the 
second  floor  is  slower  and  safer. 

It  is  also  evident  that  all  ware  dried  on  the  second  floor 
is  without  expense  for  fuel  and  further  that  the  hot  floor  can 
be  kept  at  a  maximum  operation  on  ware  that  will  dry 
safely  under  such  conditions,  so  that  the  combination  of  a  hot 
floor  and  upper  drying  floor  is  an  economy  which  should  not 
be  neglected. 


DRYING    CLAY    WARES 


In  a  number  of  yards,  waste  heat  is  drawn  from  the  cool- 
ing kilns  by  a  fan  and  distributed  through  the  building  in 
galvanized  iron  pipe  so  that  blasts  of  hot  air  can  be  turned 
on  the  stacks  of  bricks  that  have  reached  a  drying  stage, 
where  they  will  stand  rapid  finish.  This  is  the  only  use  in 
fire  brick  plants  of  waste  heat  from  cooling  kilns,  but  with 
the  advent  of  the  tunnel  dryer  in  the  fire  brick  factories,  a 
greater  use  of  this  valuable  heat  will  be  made. 

Hot  floors  are  sometimes  built  with  brick  flues  covered 
with  cast  iron  or  steel  plates.  The  steam  pressure  may  be 
so  low  that  the  leakage  through  the  lapped  joints  of  the 
plates  is  hardly  noticeable. 

The  use  of  hot  floors  is  not  uncommon  In  brick  and  tile 
plants.  The  bricks  may  be  handled  on  pallets  and  dumped 
directly  on  the  floor  as  with  fire  bricks,  or  they  may  be 
placed  on  foot  pallets  and  delivered  to  the  floor  by  lifting 
trucks,  the  bricks  remaining  on  the  pallets  until  dry — or  flat 
pallets,  lifting  cars  and  supporting  stanchions  may  be  used 
as  illustrated  under  air  drying,  or  the  dry  floors  may  be 
equipped  with  tracks  and  standard  dryer  cars.  The  latter, 
however,  would  hardly  be  considered.  The  hot  floor  is  not  the 
most  efficient  type  of  dryer,  and  if  a  car  equipment  is  to  be 
used,  a  better  type  of  dryer  should  be  adopted.  The  advan- 
tage of  the  hot  floor  is  that  it  lends  itself  readily  to  the  use 
of  pallets  and  lifting  trucks  or  lifting  cars.  A  good  feature 
of  it  is  that  after  once  in  full  operation  there  is  a  large  mass 
of  concrete  and  earth  heated  up,  which,  in  a  number  of  plants, 
suffices  to  carry  the  operation  through  the  night  without  the 
use  of  steam,  or,  in  other  words,  we  store  up  enough  heat 
in  the  day  time  to  run  the  plant  through  the  night. 

The  hot  floor  dryer  has  some  advantages  in  small  yards 
where  the  operation  is  not  continuous  throughout  the  year, 
and  where  no  waste  kiln  heat  is  available,  but  is  hardly  to  be 
recommended  for  large  capacity  plants,  except  in  the  fire 
brick  industry,  where  it  is  necessary  to  watch  the  progress 
of  the  drying. 

Compared  with  other  types  of  modern  dryers  the  cost  of 
installation  is  low,  which  is  usually  a  consideration;  the  labor 
cost  is  not  excessive,  especially  in  small  yards  where  the 
distances  are  short;  a  combination  of  brick  and  drain  tile 
very  common  in  small  yards,  works  nicely  with  the  hot  floor, 
since  the  more  easily  drying  tile  may  be  dried  on  the  upper 
floor. 


DRYING    CLAY    WARES 


CHAPTER   VIII. 
Sewer  Pipe  Floors. 

THE  DRYING  ROOMS  for  sewer  pipe,  drain  tile  and  fire- 
proofing  hardly  need  any   description.     Manufacturers 
of  drain  tile,  fireproofing  and  other  hollow  ware  that  do 
not  need  to  be  finished  after  leaving  the  machine  or  press 
are  adopting  tunnel  dryers  for  the  small  sizes  of  ware,  but 
retain  the  dry  floors  for  the  large  sizes. 

A  sewer  pipe  dryer  is  a  large  building  with  three  to  four 
floors,  including  the  ground  floor.  The  customary  plan  in  the 
past,  and  still  largely  followed,  has  the  steam  piping  under 
the  second  floor.  The  press  is  placed  to  deliver  the  ware  on 
this  floor,  and  the  large  sizes  which  have  to  be  turned  as  the 
drying  progresses  are  lowered  by  gravity  to  the  ground  floor. 
The  steam  pipes  being  overhead,  all  this  ware  is  heated  by 
radiation  from  above,  and  the  top  of  the  ware  dries  first,  as 
it  should,  and  the  finishing  work  is  done  when  the  pipes  are  in 
the  best  condition  for  this  work.  It  is  necessary  that  little 
or  no  drying  take  place  in  the  large  pipe  at  the  floor  level, 
because  the  weight  of  the  pipe  would  prevent  shrinkage  and 
the  pipe  would  crack  to  relieve  the  shrinkage  strains.  As 
soon  as  the  top  is  dry,  sufficiently  so  that  the  danger  of  crack- 
ing from  any  drying  which  might  occur  at  the  floor  level  is 
obviated,  the  pipe  is  turned,  bringing  the  bottom  to  the  top, 
and  is  then  left  on  the  floor  until  the  drying  is  completed. 
The  pipe  leaves  the  press  with  the  socket  down,  but  it  is 
turned  at  the  machine  and  placed  on  the  shod  (pallet)  with 
the  socket  up  and  so  placed  on  the  floor  for  the  initial  drying. 
The  upper  floors  are  always  slatted  where  a  single  piping 
system  is  used,  but  in  some  sections  the  floors  are  made  solid 
and  there  is  an  overhead  pipe  system  for  each  floor.  The 
slatted  floors  are  made  of  four-inch  strips  spaced  about  one- 
half  inch.  The  second  floor  in  a  single  heating  system  plant 


DRYING    CLAY    WARES  71 

is  the  most  rapid  drying  floor,  and  upon  it  is  placed,  or  should 
be  placed,  the  small  ware  which  will  stand  rapid  drying. 

The  hot  air  from  the  lower  floors  rises  through  the  slatted 
floors  to  the  upper  floors,  gathering  moisture  in  its  course  and 
finally  escapes  through  monitors  on  the  roof. 

A  frequent  annoyance  is  the  condensation  under  the  roof 
and  constant  dripping  on  the  ware.  Steep  roofs  (one-fourth 
pitch  or  more)  covered  with  shingles  will  not  "sweat,"  but 
such  roofs  are  not  regarded  with  favor  on  account  of  fire  risk. 
To  prevent  dripping  from  flat  roofs,  a  ceiling  is  put  in,  thus 
giving  air  space,  and  an  occasional  steam  pipe  under  the  roof 
maintains  a  temperature  sufficient  to  prevent  condensation. 

An  average  one-press  shop  has  a  capacity  of  from  sixty 
to  seventy-five  tons  per  day,  and  the  dry  floor  space  required 
is  from  800  square  feet  to  1,000  square  feet  per  ton  of  ware. 
This  means  from  50,000  square  feet  to  75,000  square  feet  of 
floor,  and  the  dimensions  of  a  three-story  factory  will  be  in 
round  numbers,  100x200  feet. 

It  is  desirable  to  have  the  building  rectangular  rather  than 
square;  first,  on  account  of  light,  and  second,  to  distribute 
the  ware  in  front  of  the  kilns  in  which  it  is  to  be  set. 

The  press  Is  preferably  placed  in  an  annex  midway  of 
the  length  of  the  dryer. 

The  green  ware  from  the  press  is  lowered  to  the  ground 
floor  on  gravity  drops,  and  elevated  to  the  upper  floors  on 
power  elevators.  When  dry,  the  ground  floor  product  is 
wheeled  or  trucked  direct  to  the  kilns,  and  the  upper  floor 
product  is  lowered  on  gravity  drops,  which  at  the  same  time 
return  the  empty  trucks  to  the  floor  in  question. 

In  most  instances  no  provision  is  made  for  the  admission 
of  air,  and  the  supply  depends  upon  leakage,  open  doors,  ele- 
vator shafts,  etc. 

There  is  no  data  in  regard  to  the  quantity  of  piping.  Origi- 
nally the  single  pipe  system  used  one-inch  pipe  spaced  12  to 
15  inches  under  the  entire  second  floor  except  around  elevator, 
etc.  This  would  require  from  15,000  to  18,000  feet  of  piping, 
or  5,000  to  6,000  square  feet  of  radiating  surface,  not  counting 
mains  and  headers,  verticals  and  returns,  roof  piping,  etc., 
which  materially  increase  the  radiating  surface.  It  Is  ar- 
ranged in  sections  and  all  of  it  is  not  necessarily  in  opera- 
tion at  the  same  time.  Later  plants  have  put  in  1^-inch 
pipe  without  increasing  the  spacing  beyond  15  inches,  which 
would  give  in  excess  of  6,000  square  feet  of  radiating  sur- 
face, not  counting  the  mains,  etc. 

In   determining   the    radiating  surface  required  to  heat  a 


72 


DRYING    CLAY    WARES 


given  building,  R.  C.  Carpenter  used  the  following  formula: 

NC  W 

-+G+— 


in  which  H  =  heat  units  per  degree  difference  in  temperature^ 
C  =  cubic  content  of  the  building;  G  =  glass  surface;  W  - 
wall  surface;  N  =  number  of  times  air  is  changed  per  hour. 
We  estimate  that  a  building  100x200  feet,  three  stories  high, 
has  600,000  cubic  feet  content,  15,000  square  feet  wall  surface 
and  2,700  square  feet  glass  surface.  For  winter  work,  let  us 
assume  the  outside  temperature  as  32  degrees  F.  and  the  room 
temperature  92  degrees  F. 

N  can  be  obtained  from  the  volume  of  moisture  to  be  re- 
moved. Sixty  tons  of  clay  made  into  pipe  will  contain  fifteen 
tons,  or  30,000  pounds  of  water,  which  must  be  removed  every 
twenty-four  hours.  If  the  air  enters  70  per  cent,  saturated 
and  leaves  fully  saturated,  each  cubic  foot  will  remove  .002 
pound  of  moisture,  and  therefore  there  must  be  15,000,000 
«ubic  feet  of  air  pass  through  the  building  each  day,  or  air 
in  the  building  must  be  changed 

(15,000,000) 

— =25  times  in  24  hours. 
(600,000) 

25x600,000 


24  15,000      20,000 

1-2,700-f — 1 —  =  19,810. 

55  4  10 

19,810x(92— 32)=1,188,600  B.T.U.  per  hour. 
We  add  to  the  wall  surface  the  approximate  roof  area  on  the 
the  basis  of  wood  construction.    In  ordinary  building  heating 
the  roof  need  not  be  considered,  because  usually  an  attic  in- 
tervenes between  it  and  the  rooms  to  be  heated;  but  there  is 
no  attic  in  a  sewer  pipe  plant.     Carpenter  assumes  a  radia- 
tion value  of  280  B.T.U.  per  hour  per  square  foot  of  steam 
radiating  surface: 
1,188,600 

-  —  4,245  square  feet  radiating  surface. 
280 

Assume  that  summer  conditions  are  82  degrees  F.  outside 
temperature,  air  70  per  cent,  saturated.  We  determine  in  the 
same  way  that  1,295  square  feet  of  radiating  surface  will  be 
required.  In  the  last  determination  each  cubic  foot  of  air 
takes  out  .00108  pound  of  moisture  and  27,800,000  cubic  feet 
will  be  required  daily,  or,  in  round  numbers,  the  air  in  the 
building  must  be  changed  twice  per  hour. 


DRYING    CLAY    WARES  73 

These  calculations  do  not  take  into  consideration  the  work- 
ing conditions  in  a  pipe  dryer.  We  are  heating  up  seventy- 
five  or  more  tons  of  clay  and  water  from  some  lower  tem- 
perature to  92  degrees  F.  and  evaporating  30,000  pounds  of 
water.  We  may  neglect  the  sensible  heat  in  the  mass,  so  far 
as  the  dryer  is  concerned,  because  it  presumably  heated  up 
in  preparation  and  pressing.  There  remains  30,000  pounds  of 
water  to  be  evaporated  at  92  degrees  F.  The  latent  heat  at 
92  degrees  is  1,041.1  B.T.U.  per  pound,  and  the  total  heat  re- 
quired per  day  will  be  30,000x1,040.1=31,203,000  B.T.U.,  or,  in 
round  numbers,  1,300,000  B.T.U.  per  hour.  The  radiating  sur- 
face required  will  be 

1,300,000 

=  4,643  square  feet. 

280 

Adding  this  to  the  radiation  losses,  we  find"  8,888  square  feet 
of  radiating  surface  required  for  winter  work  and  5,908  square 
feet  for  summer  work. 

The  problem  is  merely  illustrative,  and  we  have  made  no 
attempt  to  work  out  the  niceties  of  it,  which  would  only  con- 
fuse the  main  points  which  we  wish  to  bring  out. 

It  is  evident  that  a  radiating  surface  of  6,000  to  9,000  square 
feet  will  be  required,  depending  upon  climatic  conditions. 

It  is  also  evident  that  the  older  factories  with  6,000  to 
7,000  feet  of  radiating  surface  were  not  fully  efficient  under 
unfavorable  weather  conditions,  and  this  probably  accounts 
for  the  increase  in  piping  in  the  more  recent  factories.  As 
an  offset  to  this  the  older  factories  were  wood  structures,  the 
conductivity  of  which  is  less  than  one-half  that  of  brick,  and 

W  W 

—  in  the  formula  becomes  —  or  less,  depending  upon  the 
4  10 

insulation. 

There  is  no  published  data  in  regard  to  the  power  required 
to  operate  a  sewer  pipe  dryer.  One-press  shops  usually  in- 
stall three  to  four  boilers  with  a  rated  power  of  400  to  450- 
h.p.,  but  when  occasion  requires  one  of  these  can  be  cut  out 
for  cleaning  or  repairs  without  shutting  down  any  part  of  the 
operation. 

An  approximation  of  the  power  required  for  drying  may  be 
made  from  the  preceding  problems.  In  the  winter  problem 
we  have  1,188,600  B.T.U.  per  hour  to  maintain  the  factory  tem- 
perature, and  1,300,000  B.T.U.  per  hour  for  the  evaporation  of 
round  numbers,  1,300,000  B.T.U.  per  hour.  The  radiating  sur- 
the  water,  making  a  total  of  2,488,600  B.T.U.  per  hour.  A 
boiler  horse  power  is  rated  at  30  pounds  of  water  from  100 


DRYING    CLAY    WARES 


degrees  F.  steam  at  70  pounds  pressure,  and  this  requires 
33,450  B.T.U.  The  boiler  horse  power  therefore  for  the  above 
requirement  would  be  75,  but  this  is  unquestionably  too  low. 
There  are  boiler  losses,  pipe  losses  in  transmission  to  the 
dryer,  frictional  losses  in  moving  the  steam  and  water  through 
the  piping,  leakage  losses,  steajn  losses  in  the  return  mains 
and  vacuum  pump.  Allowing  10  per  cent,  for  these  losses 
brings  the  boiler  requirement  for  drying  alone  to  about  83-h.p. 
We  have  no  data  to  determine  whether  10  per  cent,  is  even 
an  approximation  of  these  losses.  The  boiler  radiation  losses 
alone  have  been  carefully  calculated  and  even  determined 
direct,  and  when  the  boilers  are  properly  protected,  are  reck- 
oned at  4  per  cent.  Besides  the  actual  work  of  drying,  there 
is  the  heat  required  for  the  plaster  and  molding  rooms,  clay 
preparing  room, .press  room,  etc.,  which  cannot  be  separated 
from  the  actual  boiler  requirement. 

Waste  heat  from  cooling  kilns  is  not  largely  used  in  sewer 
pipe  drying.  Three  or  more  plants  were  built  to  use  waste  heat 
in  connection  with  steam  piping,  but  in  one  instance  at  least 
the  method  was  abandoned.  The  steam  piping  was  placed  under 
the  second  floor,  as  usual  in  older  plants.  Figs.  40  and  41  show 
the  methods  of  introducing  and  distributing  the  air  in  two  fac- 
tories. A  plate  fan  collects  the  hot  air  fro.m  the  cooling 
kilns,  or  from  sectional  steam  heating  coils,  and  forces  it  into 
the  building  through  galvanized  iron  pipes  (Fig.  41).  From 
the  verticals,  under  each  floor,  are  four  small  distributing 
pipes,  so  placed  and  of  such  an  extent  that  each  riser  suffices 
for  six  sections  of  the  floor,  or  about  1,500  square  feet.  Fig. 
40  shows  the  distributing  outlet  in  another  plant.  In  this  in- 
stance the  heat  was  distributed  under  the  lower  floor  only. 
Each  perforated  pipe  was  enclosed  by  a  galvanized  iron  pipe, 
also  perforated  to  mate  with  the  perforations  in  the  inner  pipe. 
When  it  was  desired  to  shut  off  the  heat  in  any  section,  the 
outer  pipe  was  turned  so  the  holes  missed  connection.  We 
believe  the  hot  air  system  in  the  latter  plant  was  abandoned, 
and  also  in  one  plant  using  the  distributing  pipe  system.  The 
difficulty  in  such  hot  air  system  is  to  get  even  distribution  of 
the  heat.  Sections  of  the  floor  immediately  over  and  adjacent 
to  the  heating  pipes  will  get  greater  heat  than  intermediate 
sections,  and  if  the  clay  is  at  all  tender  there  will  be  excessive 
loss  in  cracked  ware  in  the  vicinity  of  the  distributing  pipes. 

Another  method  of  working  out  this  waste  heat  problem 
which  is  in  successful  operation,  is  to  have  a  deep  basement 
under  the  lower  floor,  with  steam  pipes  under  the  first  floor. 
The  hot  air  from  the  kilns  is  simply  blown  into  the  basement, 


DRYING    CLAY    WARES 


Figure  40. 


DRYING    CLAY    WARES 


in  which  there  is  ample  space  for  diffusion  before  its  passage 
among  the  steam  pipes  and  up  through  the  slatted  floor. 
There  could  be  no  marked  changes  in  temperature  from 
one  section  of  the  floor  to  the  next,  and  one  could  easily 
learn  what  portion  of  the  floor  must  be  reserved  for  tender 
drying  ware.  No  ware  is  placed  on  the  basement  floor. 

This  recalls  another  point  previously  mentioned  in  regard 
to  sewer  pipe  plants;  namely,  that  in  very  few  plants  is  any 
specific  provision  made  for  ventilation.  Leakage  is  relied 
upon  for  inlet  air  and  windows  in  monitors  for  outlet. 

The  use  of  a  fan  as  above  mentioned  insures  any  desired 
volume  of  air,  and  it  is  practical  to  distribute  it  uniformly 
through  the  several  floors.  Natural  exhaustion  at  the  top  is 
perhaps  satisfactory,  but  we  believe  a  forced  exhaustion  by 
suction  fans  would  be  better. 

Many  people  have  the  erroneous  idea  that  moisture  ladened 
air  is  heavier  than  dry  air  and  attempts  have  been  made  to 
adapt  down  comer  ventilators.  This  is  a  mistake.  The  more 
moisture  air  takes  up  the  lighter  it  becomes,  and  completely 
satured  air  has  the  least  weight  per  cubic  foot,  temperature 
of  course  remaining  the  same.  Water  vapor  is  lighter  than 
air,  and  instead  of  being  taken  up  by  the  air,  displaces  it.  A 
cubic  foot  of  dry  air  when  saturated  with  vapor  will  occupy 
more  than  a  cubic  foot  of  space,  and  the  moisture  has  a  lower 
specific  gravity.  As  air  cools,  however,  it  becomes  heavier, 
and  the  cooling  effect  should  be  counteracted  by  secondary 
heating,  which  at  the  same  time  prevents  condensation  of  the 
moisture.  The  air  rising  from  the  first  floor,  whether  the 
floor  be  heated  or  not,  is  partly  saturated  with  moisture. 
Passing  the  pipes  under  the  second  floor,  it  becomes  heated 
and  its  capacity  for  moisture  correspondingly  increased.  The 
ware  on  the  second  floor  gives  up  moisture  to  the  air  with- 
out saturating  it,  but,  in  passing  through  the  third  floor  and 
the  fourth  floor,  both  in  taking  up  moisture  and  cooling,  the 
air  becomes  saturated,  and  may  have  little  or  no  capacity  for 
moisture  in  the  upper  floor,  and,  because  of  its  greater  weight 
through  cooling,  it  acts  as  a  blanket  or  damper.  With  steam 
pipes  under  the  third  and  fourth  floors,  we  maintain  the  tem- 
perature and  the  air  is  lighter  in  consequence  of  the  vapor 
and  becomes  lighter  with  each  increase  of  vapor.  Of  the 
capacity  of  the  air  to  take  up  moisture,  25  per  cent  may  be 
used  in  the  first  floor,  25  per  cent  in  the  second,  25  per  cent 


DRYING    CLAY    WARES  77 

in  the  third,  and  complete  saturation  reached  in  the  fourth. 
The  air  becomes  more  buoyant  as  it  rises  and  we  only  need 
to  maintain  the  temperature  by  steam  pipes  under  the  roof 
until  the  air  can  reach  the  exits,  where  it  will  discharge  itself 
fully  ladened  with  moisture  and  carrying  materially  more 
moisture  than  if  heated,  only  by  a  single  system  of  piping 
under  the  first  or  second  floor. 

We  recall  only  one  instance  in  which  a  tunnel  dryer  for 
small  ware  was  used  in  connection  with  dry  floors  for  large 
pipe,  and  the  operation  was  not  satisfactory.  On  drain  tile, 
however,  which  does  not  have  to  be  rolled  and  finished,  the 
modern  plant  uses  a  tunnel  dryer  for  small  sizes  and  the 
dryer  floors  for  large  tile. 


7S 


DRYING    CLAY    WARES 


CHAPTER   IX. 
Periodical   Dryers. 

A    NY  DRYER,  in  which  the  ware  remains  stationary  dur- 
A    ing  the  drying,  is  periodical,  and  the  open  air  and  the 
<*»•  kiln  dryers  previously  described  belong  in   this   class. 
There  are  two  distinct  advantages  in  the  periodical  dryer: 

1.  It  is  adapted  to  the  use  of  lifting  cars  or  trucks,  or  a 
conveyor  system  for  delivery  of  the  ware  into  the  dryer  and 
removing  it  when  dry. 

2.  In  some  types  it  permits  slow  heating  up  with  as  little 
or  as  much  air  as  may  be  desired,  and  when  the  ware  has 
reached  a   condition   where   it  will   stand   rapid   drying,   the 
temperature  may  be  advanced  to  any  degree  within  the  limits 
of  the  heating  equipment  and  any  required  volume  of  air  may 
be  introduced. 

Steam  Pipe  Dryers. 

A  combined  radiation  and  convection  periodical  dryer  con- 
sists of  a  series  of  tunnels  with  steam  pipes  under  the  tracks 
or  runways,  and  sometimes  along  the  sides.  With  the  air 
inlets  and  outlets  closed,  the  ware  can  be  heated  up  by  radia- 
tion with  no  convection  except  such  air  currents  as  may  be 
occasioned  by  leakage.  When  the  ware  has  been  heated  up 
and  thus  put  through  any  desired  degree  of  humidity  treat- 
ment, the  air  vents  may  be  opened  and  the  drying  finished 
rapidly.  With  the  steam  pipes  in  the  dryer  tunnels  we  have 
no  radiation  loss  except  that  from  the  dryer  building  itself, 
which  is  unavoidable  in  any  dryer. 

As  will  be  seen  under  the  description  of  the  progressive 
type  of  waste  heat  dryers,  a  large  volume  of  air  is  required 
to  bring  in  sufficient  heat  to  do  the  drying. 

In   the   periodical   steam    dryer   we   need   very   little    air, 


DRYING    CLAY    WARES  79 

merely  sufficient  to  sweep  out  the  moisture  as  it  is  developed. 
The  problem  figures  as  follows: 

An  ordinary  tunnel  is  100  feet  long  and  holds  7,000  brick. 
If  each  brick  contains  one  pound  of  water,  and  the  drying 
period  is  twenty-four  hours,  we  have  7,000  pounds  to  evapo- 
rate and  remove  from  the  dryer  in  that  period.  Assume  that 
the  air  enters  at  60  degrees  F.  and  70  per  cent,  saturated, 
then  from  the  table  of  vapor  capacities,  page  19,  we  find 
that  each  cubic  foot  of  air  brings  in  .00059  pounds  of  moisture. 
If  the  dryer  is  heated  to  200  degrees  F.,  which  can  easily  be 
done  with  high  pressure  live  steam,  the  incoming  air  and 
moisture  expands  to 

1  x 


491+ (60-32)         491+ (200-32) 
From  which  we  find  x  =  1.27 

from  Charles'  law  that  the  volume  of  gas  is  proportional  to 
the  absolute  temperatures  and  a  cubic  foot  of  the  expanded 
air  will  contain 

.00059 

—  =  .00047  pounds  of  moisture. 
1.27 

From  the  same  table  of  vapor  capacities  we  find  that  satu- 
rated air  at  200  degrees  contains  .03024  pounds  per  cubic 
foot,  and  therefore  each  cubic  foot  of  air  will  remove  from 
the  bricks  .03024— .00047=.02977  pounds.  To  remove  1,000 
pounds  of  water  in  twenty-four  hours,  we  must  have  in  round 
numbers  33,600  cubic  feet  of  air  at  200  degrees,  or  26,450 
cubic  feet  at  60  degrees. 

We  determine  the  radiation  loss  as  follows: 
A  tunnel  is  3  feet  6  Inches  wide,  4  feet  8  inches  high  and 
100  feet  long,  and  contains  1,630  cubic  feet,  from  which  we 
deduct  346  cubic  feet  for  brick  and  cars,  leaving  1,288  cubic 
feet,  and  since  the  air  required  per  tunnel  for  drying  is 
(7X33600)  235,200  cubic  feet,  the  air  in  the  tunnel  must  be 
changed  (235,200/1288)  one  hundred  and  eighty-two  times  in 
twenty-four  hours,  or  7.6  times  per  hour. 

In  a  battery  of  six  tunnels  there  will  be  867  square  feet 
of  exposed  wall,  233  square  feet  of  iron  doors,  and  2,500 
square  feet  of  well-insulated  roof,  the  radiation  from  which 
may  easily  be  reduced  to  one-twentieth  that  of  glass.  The 
radiation  from  iron  is  1.1  times  that  from  glass,  and  the 
other  factors  have  been  given  in  the  discussion  of  sewer  pipe 
drying. 


80  DRYING    CLAY    WARES 

Using  Carpenter's  formula,  we  get  for  radiation  loss  from 
one  tunnel 

7.6  X  1288         233  X  1.1         867          2500 


55  6  4  X  6      20  X  6 

=  277  B.  T.  U.  per  degree  difference  of  temperature  per  hour. 
The  difference  in  temperature  is  200  —  60  =  140,  and  the  to- 
tal radiation  loss  per  tunnel  will  be  277  X  140  =  38,780  B.  T. 
U.  per  hour,  or  930,720  B.  T.  U.  in  24  hours,  or  930,720/7  = 
132,960  B.  T.  U.  per  1,000  pounds  of  water  evaporated. 

It  may  be  noted  from  what  follows  that  the  radiation  loss 
as  determined,  is  about  9  per  cent,  of  the  total  heat  require- 
ment. It  has  been  customary  to  estimate  dryer  losses  at  10 
per  cent.,  and  it  was  also  formerly  customary  to  estimate  kiln 
radiation  losses  at  10  per  cent.,  but  some  commercial  tests 
have  shown  kiln  radiation  losses  up  to  70  per  cent.  It  is 
probable  that  a  well-insulated  dryer  will  have  a  radiation  loss 
less  than  10  per  cent.,  and  our  calculations  confirm  this. 

The  value  of  such  calculations  in  which  there  are  a  num- 
ber of  assumed  factors  may  be  questioned,  but  we  hold  that 
any  calculation  is  a  better  basis  for  the  exercise  of  one's 
judgment  than  a  mere  guess. 

In  the  drying  we  have  the  following  heat  requirement  per 
thousand  bricks: 

1,000  pounds  of  iron  to  be  heated  from  60°  to  200°. 

6,000  pounds  of  clay  to  be  heated  from  60°  to  200°. 

26,450  cubic  feet,  or  26,450  X  .075  =  1,984  pounds  of  air  to 
be  heated  from  60°  to  200°. 

1,000  pounds  of  water  to  be  evaporated  at  200°. 

15.6  pounds  of  water  vapor  originally  in  the  air,  to  be 
heated  from  60°  to  200°. 

The  specific  heat  of  iron  is  taken  as  .12;   of  clay,  .2;   the 
sensible  and  latent  heat  required  in  changing  water  at  60°  to 
vapor   at   200°    is    1118;    the    mean    specific    heat    of   a    gas 
is  k  +   s    (T   +  t).     For  water  vapor  the  value  of  "k"   is 
.42  and   of  "s"   is   .0001,  while  for  air  "k"   is   .234   and   "s" 
.000012.    "T"  and  "t"  are  the  temperatures  less  32°. 
1000  X  .12  X  140  =  ...................  16)800  B.  T  U. 

6000  X   .2   X   140  =  .........................     168>000  B.  T.  U. 

1984  X  [.234  +  .000012  (168  +  28)]  140  =.  .  .  65,650  B.  T.  U. 
1000  X  1118  (Heat  in  vapor  at  200  —  60)=  1  118  000  B  T  U 
15.6  X  [.42  +  .0001  (168  +  28)]  140  =.  .  .  960  B.  T.  U. 


1,369,410  B.  T.  U. 
Adding  the   radiation  loss   as   previously   determined,   we 


DRYING    CLAY    WARES  81 

have  a  total  heat  requirement  of  1,502,370  B.  T.  U.  per  thou- 
sand bricks  dried. 

The  next  problem  is  to  determine  the  amount  of  piping  re- 
quired. 

Carpenter's  factor  of  280  B.  T.  U.  radiation  per  square 
foot  of  radiating  surface  may  be  used,  but  it  is  only  applica- 
ble to  problems  in  which  the  current  of  air  passing  over  the 
piping  is  very  slow. 

The  rule  of  thumb,  in  which  the  quantity  of  heat  given 
off  from  steam  pipes  varies  from  1.25  to  3.25  B.  T.  U.  per 
square  foot  per  hour  per  degree  difference,  gives  us  a  basis 
upon  which  to  exercise  our  judgment,  but  it  makes  no  separa- 
tion of  radiation  and  convection. 

The  amount  of  heat  given  off  depends  upon  radiation  and 
convection.  The  radiation  value  is  constant  for  each  temper- 
ature, but  the  convection  value  depends  upon  the  size  of  the 
pipe  and  the  number  of  changes  of  air. 

It  is  evident,  as  will  be  seen  in  the  discussion  of  indirect 
heating,  that  with  increased  circulation  we  get  greater  con- 
densation and  in  consequence  greater  heat  return  from  a 
given  amount  of  piping.  Richards'  "Metallurgical  Calcula- 
tions" adopts  the  basis  that  the  heat  given  off  varies  as  the 
square  roots  of  the  velocities  and  uses  the  formula, 


A  familiar  illustration  of  the  advantages  of  circulation  is 
that  of  a  heater  which  is  insufficient  to  heat  a  closed  room  by 
natural  circulation,  but  which  becomes  sufficient  if  a  fan  is 
placed  to  force  the  air  in  contact  with  it.  There  is  no  change 
in  the  conditions  except  greater  circulation,  which  increases 
the  convected  heat  taken  from  the  heater. 

We  can  readily  calculate  the  radiation  and  convection 
values  for  natural  ventilation,  but  when  we  attempt  to  cor- 
rect these  values  for  increased  changes  of  air  we  get  into  dif- 
ficulties which  make  the  calculations  of  little  or  no  value. 

We  have  the  assurance,  however,  that  the  piping  required 
for  natural  ventilation,  other  things  being  equal,  is  in  excess 
of  that  required  for  greater  velocities  of  air  among  the  piping. 

We  have  found  the  following  method  the  most  satisfactory 
in  determining  radiation  and  convection  losses.  It  is  based  on 
Peclet's  experiments,  upon  Newton's  law  of  radiation,  and 
upon  Dulong's  corrections  for  Newton's  law.  The  data  for  the 
calculation  will  be  found  in  one  form  or  another  in  several 
treatises  on  heating  and  ventilating  (Box  "Treatise  on  Heat," 


82 


DRYING    CLAY    WARES 


Kent's  "Mechanical  Engineer's  Pocket  Book,"  Carpenter's 
"Heating  and  Ventilating  Buildings"),  but  as  a  rule  it  is  not 
in  convenient  form  for  ready  calculation. 

According  to  Newton's  law,  the  radiation  from  steam  pip- 
ing varies  with  the  difference  in  temperature,  namely,  .64  X 
difference  in  temperature  =  radiation,  but  this  has  been  found 
incorrect  for  wide  differences  in  temperature. 

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The  factors  for  correction  are  given  in  the  following  table, 
and  it  will  be  sufficiently  accurate  for  practical  purposes  to 
take  the  factor  nearest  to  the  required  temperature  and  de- 
termined difference  of  temperature. 

Chart  No.  1  gives  the  convection  losses  of  different  sizes 
of  piping  for  one  degree  temperature  difference  per  square 


DRYING    CLAY    WARES 


s:; 


foot  of  surface  and  Chart  No.  2  gives  the  factors  for  correc- 
tion necessary  in  wide  differences  of  temperature.* 

In  .our  problem  we  have  a  temperature  difference  of  247°, 
the  nearest  to  which  In  the  first  column  of  the  table  is  243°. 
The  factor  for  this  difference  of  temperature  and  for  an  air 
temperature  of  59°,  which  is  nearest  our  assumed  air  tem- 
perature (60°),  we  find  to  be  1.88.  The  heat  given  up  by  ra- 


l.iO 


Chart  lloJ 
Con  v  ec  t  i  on 

From  Hortjonfat 


for  Ct   Temp    Difference 


,3  4-  5  6  8 

Oft  side   Diameter  Of '  Pt/ie/n  /nche-s 


diation  then  is  .64  X  1.88  X  247  =  297  B.  T.  U.  By  interpo- 
lations to  get  the  exact  factor,  we  find  the  radiation  to  be  301 
B.  T.  U.,  which  shows  that  the  table  is  sufficiently  extended 


*Our  problem  is  based  on  the  use  of  live  steam  at  60  Ibs. 
pressure,  which  has  a  temperature  of  307°  F. 


84 


DRYING    CLAY    WARES 


for  practical  problems.  Figuring  the  same  problem  from  data 
given  in  "Heating  and  Ventilating  Buildings"  (Carpenter), 
we  get  322  B.  T.  U. 

Convection  is  determined  from  the  charts  No.  1  and 'No.  2. 


-fOO 


C/i  aff  Tlo  2 
r  ro>-ffect<jct, on  TeyDutenajLaw  Of  Convecfi.in 


-.'300- 


0:5 

Facto 


Our  problem  assumes  that  1-inch  pipe  will  be  used,  which  has 
1.315  inches  outside  diameter. 

From  Chart  No.  1  we  find  that  piping  with  diameters  of 


DRYING    CLAY    WARES  85 

1.3  inches  strikes  the  curve  opposite  .97  heat  units  lost.  From 
Chart  No.  2  we  find  that  247  difference  in  temperature  pro- 
jected to  the  curve  and  thence  to  the  base  co-ordinate  gives  a 
factor  of  1.73. 

The  convection  loss,  therefore,  is  .97  X  1.73  X  247  =  414 
B.  T.  U.  From  the  data  given  by  Carpenter,  the  convection 
value  for  the  assumed  conditions  is  380  B.  T.  U.  The  total 
radiation  and  convection  loss  from  our  table  and  charts  is  711 
B.  T.  U.  and  from  Carpenter's  data  702  B.  T.  U. 

From  our  table  and  charts  it  is  possible  to  figure  the  prob- 
lem for  any  difference  of  temperature,  for  any  outside  air 
temperature  and  for  any  size  of  piping,  and  the  results  are 
sufficiently  accurate  for  any  practical  problem.  The  result  we 
obtained  above,  reduced  to  B.  T.  U.  per  degree  difference, 
gives  us  2.87  B.  T.  U.  per  square  foot  per  hour. 

With  a  heat  development  of  711  B.  T.  U.  per  square  foot 
per  hour,  or  17,064  B.  T.  U.  per  day,  we  will  require  for  the 
estimated  heat  requirement  of  1,502,370  B.  T.  U.  for  1,000 
bricks,  88.2  square  feet  of  heating  surface. 

The  average  capacity  of  a  tunnel  is  7,000  bricks,  and  there- 
fore 616  square  feet  of  heating  surface  will  be  required  per 
tunnel,  or  1,848  lineal  feet  of  1-inch  pipe. 

Low-pressure  steam — let  us  say  5  pounds — will  have  a  tem- 
perature of  227°  F.  and  the  difference  in  temperature  will  be 
167°.  From  the  table  of  factors  we  find  that  the  factor  for  a 
temperature  difference  of  171°  and  air  temperature  of  59°  is 
1.575  and  the  heat  given  off  by  radiation  will  be  1.575  X  .64  X 
167  =  168  B.  T.  U. 

From  Chart  No.  1  we  get  for  1-inch  pipe  the  factor  .97,  as 
before,  and  from  Chart  No.  2,  for  a  temperature  difference  of 
167,  we  get  the  factor  1.58.  The  convected  heat  then  is  .97  X 
1.58  X  167  =  256  B.  T.  U.,  making  the  total  heat  loss  424  B. 
T.  U.  per  square  foot  per  hour,  or  2.54  B.  T.  U.  per  degree  dif- 
ference per  square  foot  per  hour. 

The  piping  required  under  such  condition  would  be 

1,502,370 
—  147.6 


424  X  24 

square  feet  per  thousand  bricks,  or  1,033  square  feet  per  tun- 
nel, or  3,099  lineal  feet  of  1-inch  pipe  per  tunnel. 

The  above  determinations  of  convected  heat  are  for  nat- 
ural ventilation,  and  do  not  take  into  consideration  the  in- 
crease which  comes  from  increased  circulation  essential  in 
natural  draft  dryers.  The  solution  of  the  problem  for  natural 
draft  dryers  requires  too  many  uncertain  assumptions  to  give 
results  of  any  value,  as  the  following  discussion  will  show: 


DRYING    CLAY    WARES 


In  a  coil  heater  we  space  the  pipes  2%  inches  and  the  free 
area  will  be  1  435  inches.  If  we  have  eight  pipes  in  a  row  we 
may  count  nine  spaces  or  12.915  inches  of  free  area  in  the 
row  and  for  a  square  foot  of  free  area  we  must  have  a  sec- 
tion' (144/12.915)  11.15  inches  long,  in  which  there  will  be 
11 15  X  8  =  89.2  inches  of  piping,  which  is  equivalent  to  2.48 
square  feet  of  heating  surface.  We  require  per  tunnel  26,450 
(page  79)  X  7  =  185,150  cubic  feet  of  air  per  day,  or  7,  (15 
cubic  feet  per  hour.  If  the  tunnel  is  100  feet  long  we  have 

100  X  12  X  12.915 

=  107.6 

144 
square  feet  of  free  area.    The  velocity  of  the  air  is 

7715 

=  71.7 

107.6 

cubic  feet  per  hour  through  each  square  foot  of  free  area.  A 
cubic  foot  of  air  requires  .018  B.  T.  U.  to  raise  the  tempera- 
ture one  degree— the  formula  is  [.018  X  .0000009  (T  +  t)  ] 
(T  _  t)— and  71.7  cubic  feet  will  require  1.29  B.  T.  IT.  to 
raise  the  temperature  one  degree. 

Under   natural   ventilation   we   have,   as   previously   deter- 
mined, 414  (page  84)  B.  T.  U.  convection  value,  or  since  each 
square  foot  of  free  area  has   2.48  square  feet  of  piping,  we 
have  available  1,028  B.  T.  U., 
1028 

1.29 
which  is  an  impossible  temperature  from  the  piping. 

It  becomes  evident  at  a  glance  that  the  volume  of  air  re- 
quired to  carry  out  the  moisture  is  not  sufficient  to  carry  the 
heat  required  in  the  drying,  making  full  allowance  for  the 
heat  obtained  direct  from  radiation.  It  follows  that  since  the 
incoming  air  cannot  take  enough  heat  from  the  piping  to 
meet  the  dryer  requirement,  there  must  be  circulation  within 
the  dryer  by  which  the  hot  air,  after  being  cooled  by  the  ware 
and  by  the  evaporation  of  the  water,  returns  repeatedly  to  the 
piping  to  again  become  heated,  and  meanwhile  there  is  escap- 
ing from  the  dryer  a  quantity  of  air  equivalent  to  that  enter- 
ing. The  purpose  of  this  discussion  is  to  bring  up  the  ques- 
tion of  convection  relative  to  the  number  of  changes  of  air. 
We  have  figured  the  heat  values  for  natural  ventilation,  and 
with  any  increased  velocity  of  the  passage  of  the  air  over  the 
heating  surface  the  heat  loss  from  the  piping  due  to  convec- 
tion increases  and  the  area  of  heating  surface  correspond- 
ingly decreases.  If  we  were  introducing  sufficient  air  for  the 


DRYING    CLAY    WARES  87 

heat  requirement  at  a  single  passage  through  the  piping,  then 
we  could  determine  the  velocities  for  the  assumed  conditions 
and  for  natural  ventilation  from  the  formula, 


2  + 


\ 


and  could  determine  within  a  practical  degree  of  accuracy  the 
increased  heat  loss  from  the  piping  in  consequence. 

In  the  problem  in  question  the  quantity  of  air  is  such  that 
it  attains  a  maximum  temperature  before  passing  all  the 
heating  surface,  and  because  of  this  cannot  increase  the  con- 
vected  heat. 

The  circulating  air  becomes  the  factor  from  which  we 
must  figure  any  increased  convection,  and  as  this  will  have  a 
higher  temperature  than  the  entering  air,  we  must  base  any 
calculations  on  lower  differences  of  temperature,  which  means 
less  convected  heat  per  square  foot  of  piping  per  hour,  and  in 
consequence  more  piping  to  deliver  the  requisite  amount  of 
heat  We  cannot  determine  the  velocity  of  the  circulating  air, 
nor  the  efficiency  of  its  contact  with  the  piping,  nor  its  tem- 
perature upon  its  return  to  the  piping.  It  is  not  possible, 
therefore,  to  figure  the  problem  without  several  uncertain  as- 
sumptions, and  it  is  best  to  determine  the  piping  by  the 
method  previously  set  forth  from  data  which  can  readily  be 
determined,  and  beyond  this  to  be  governed  by  one's  judg- 
ment or  experience  in  similar  equipment,  bearing  in  mind 
that  as  we  increase  the  volume  of  air  or  the  circulation,  we 
increase  convection  and  decrease  piping,  and  as  we  decrease 
difference  of  temperature  between  the  air  and  the  steam  we 
decrease  convection  and  increase  piping. 

We  have  considered  only  actual  steam  temperatures  in 
the  piping,  but  there  should  be  some  allowance  made  for  loss 
in  transmission  from  steam  to  piping,  and  for  conduction 
through  the  piping.  It  is  possible  to  calculate  these  losses 
under  assumed  conditions,  but  an  allowance  of  3  to  5  de- 
grees in  the  temperature  difference  will  usually  fully  cover 
such  losses. 

Figures  41  and  42  show  plan  and  sectional  elevations  of  a 
steam  pipe  dryer  which,  as  shown  and  with  various  modifica- 
tions, has  found  frequent  use. 

A  double  row  of  steam  pipes  are  placed  at  the  floor  level 


88 


DRYING    CLAY    WARES 


under  the  cars  of  bricks,  and  a  single  row  is  placed  along  the 
walls  on  each  side  of  each  tunnel.  The  dryer  is  roofed  as 
shown  and  down  comer  flues  are  inserted  in  the  walls  at  in- 
tervals. The  flues  are  connected  with  cross  horizontal  flues 
at  the  bottom,  forming  an  inverted  T,  which  extends  under 
the  floor  piping.  The  air  supply  enters  through  these  flues. 
Alternating  with  these  air  inlets  are  stacks  in  the  roof  of  the 


Figure  41. 

tunnels  and  extending  through  and  above  the  protecting  roof. 

It  will  be  noted  that  one  feature  of  this  dryer  is  that  the 
air  is  taken  from  the  space  between  the  tunnel  roof  and  pro- 
tecting roof,  and  it  is  heated  to  the  extent  of  any  radiation 
from  the  tunnel  roof. 

As  much  as  3,200  lineal  feet  of  one-inch  pipe  is  used  in 
each  tunnel  100  feet  long,  which  is  a  greater  heating  surface 
than  that  determined  by  our  estimate,  but  it  must  be  remem- 


DRYING    CLAY    WARES 


bered  that  actual  conditions  can  only  determine  the  amount  of 
piping.  We  find  in  use  some  dryers  with  less  piping  than  the 
theoretical  amount  calculated  by  us,  and  some  with  more,  all 
dependent  upon  the  work  to  be  done  and  the  time. 

The  dryer  shown  is  properly  periodical,  but  where  the  clay 


Figure  42. 

will  stand  the  severe  treatment,  the  dryer  is  operated  pro- 
gressively. In  the  periodical  dryer  the  steam  is  shut  off  until 
the  tunnel  is  filled,  and  is  then  turned  on  and  the  heat  raised 
as  the  ware  will  stand  it.  If  the  clay  is  very  tender  drying, 
the  air  inlets  and  stacks  may  be  closed  until  the  ware  is 


90 


DRYING    CLAY    WARES 


DRYING    CLAY    WARES 


heated  up  and  in  a  safe  condition  to  stand  the  admission  of 
air,  and  only  such  amount  of  air  should  pass  through  the  dryer 
as  may  be  needed  to  carry  out  the  moisture. 

In  operating  the  dryer  progressively,  cars  of  freshly  made 
ware  are  put  in  at  the  receiving  end  as  fast  as  cars  of  dried 
ware  are  taken  from  the  delivery  end.  The  •  steam  is  on  all 
the  time,  and  the  ware  from  the  machine  enters  at  once  a 
drying  atmosphere  having  temperatures  of  200  to  250  degrees 


Figure  44. 


F.,  which  many  clays  will  not  stand.  Another  objection  to  the 
progressive  operation  is  that  it  would  be  difficult  to  adjust 
the  inlets  and  outlets  so  that  in  each  part  of  the  tunnels  long- 
itudinally there  will  be  the  economical  relation  between  air 
and  moisture,  and  in  consequence  complete  saturation  is  not 
attained. 

Figures  43  and  44  show  plan  and  sectional  elevations  of  a 
periodical  steam  dryer  designed  exclusively  for  tender  drying 
clay.  The  piping  is  arranged  in  three  rows  and  the  connec- 


92 


DRYING    CLAY    WARES 


tions  are  such  that  each  row  is  independent.  No  effort  is 
made  to  control  the  individual  air  inlets  except  that  the  roof 
space  from  which  these  inlets  draw  the  air  is  entirely  en- 
closed, and  when  desired  they  can  only  get  such  supply  as 
may  be  derived  from  leakage.  It  is  not  necessary,  however, 
to  have  any  control  of  the  inlets.  The  outlets  are  controlled 
by  slide  dampers,  which  can  be  operated  from  either  end  of 
the  dryer. 

As  soon  as  a  tunnel  is  filled  with  ware,  the  slide  dampers 
to  the  flue  leading  to  the  stacks  are  closed,  and  steam  is 
turned  into  the  bottom  row  of  pipes,  followed  by  the  second 


Figure  45. 

and  third  row  as  experience  may  determine.  Thus  the  ware  is 
heated  up  in  a  humid  atmosphere,  which  has  proven  very 
effectual  as  a  preliminary  treatment  in  drying  tender  clays 
Following  this  humidity  treatment  the  Me  dampers  a?e 
diawn,  slightly  or  fully,  as  the  ware  may  require,  and  Se 

FifuPre°C4e,     h^  *  ^  flrSt  d6SCribed  type  of  stea^  dryer 
in  several  S^jTVif  Pri0diCal  Steam  dryer  which  ^  used 

r 


DRYING    CLAY    WARES  93 

taining  approximately  14,000  bricks  in  the  length  of  the  tun- 
nel, or  110  feet.  At  intervals  in  the  roofs  of  the  tunnels  are 
openings  into  inclined  cross  ducts  and  the  latter  connect  with 
galvanized  iron  ducts  in  the  outside  walls,  which  in  turn  lead 
under  the  piping.  The  moisture  ladened  air  rising  from  the 
bricks  passes  through  the  openings  in  the  tunnel  roof,  thence 
to  the  flues  in  the  outside  walls,  where  it  is  presumed  to  be- 
come cooled,  the  moisture  condensed  and  carried  away  by  a 
drip  at  the  ground  level.  It  is  assumed  that  the  air  in  this 
way  renews  its  ability  to  take  up  moisture,  which  is  true  to 
whatever  extent  it  becomes  heated  in  passing  among  the 
steam  piping,  but  it  must  be  evident  to  any  one  that  the  air 
thus  returned  to  the  dryer  is  fully  saturated  all  the  time  while 
the  outside  air  on  an  average  will  not  be  more  than  75  per 
cent,  saturated.  No  heat  is  conserved,  because  the  air  re- 
turned to  the  dryer  has  the  same  temperature  as  outside  air. 
Possibly  the  inventor  had  in  mind  that  the  latent  heat  in  the 
vapor,  which  is  nearly  75  per  cent,  of  all  the  heat  required, 
would  be  returned  to  the  air  entering,  but  instead  it  is  given 
to  the  outside  air. 

Under  progressive  dryers  will  be  described  one  which  act- 
ually gives  back  to  the  dryer  the  heat  value  of  the  latent  heat 
in  the  vapor  and  the  additional  heat  required  is  that  necessary 
to  maintain  the  dryer  losses  exclusive  of  the  evaporation  of 
the  water. 

In  the  dryer,  under  discussion  a  small  air  duct  extends 
along  the  wall  on  either  side  of  each  tunnel  inside  the  tunnel. 
These  ducts  connect  with  the  outside  air  at  the  ends  of  the 
tunnels  and  have  frequent  inlets  into  the  tunnels  in  their 
length.  The  purpose  of  these  is  to  supply  leakage  losses,  and 
to  restore  the  air  which  is  carried  out  by  small  stacks  at  the 
end  of  the  dryers. 

So  long  as  clayworkers  continue  to  build  dryers  which  are 
palpably  wrong  in  principle,  it  is  evident  to  us  that  they  need 
instruction  in  the  principles  that  govern  drying. 

One  point  possibly  favorable  to  the  above  described  dryer 
is  that  it  might  have  some  application  as  a  humidity  dryer. 
If  we  had  absolutely  dry  air  at  a  temperature  which  would 
give  it  a  vapor  capacity  of  "J,"  let  us  say,  and  air  containing 
moisture,  but  at  such  a  temperature  that  it  also  had  an 
additional  vapor  capacity  of  "J,"  will  the  latter  be  a 
safer  drying  medium  than  the  former?  This  is  affirmatively 
answered  by  some,  and  numerous  instances  are  cited 
where  steam  injected  into  the  hot  air  at  the  delivery 
end  of  the  tunnels  has  overcome  considerable  loss  in 


94  DRYING    CLAY    WARES 

the  drying.  If  the  temperature  is  below  212  degrees 
one  can  readily  see  how  the  steam  could  be  of  bene- 
fit by  reducing  the  rate  of  drying  through  increase  in 
the  degree  of  saturation,  but  under  such  conditions  lower 
temperatures  should  accomplish  the  same  purpose,  yet  do  not. 
If  the  temperature  is  above  212  degrees,  the  addition  of  steam 
theoretically  could  not  affect  the  rate  of  drying,  since  above 
the  boiling  point  the  capacity  of  the  air,  if  we  may  so  express 
it,  becomes  infinite,  or,  in  other  words,  so  long  as  the  steam 
can  escape  we  cannot  lower  the  drying  rate  by  the  introduc- 
tion of  more  steam.  We  do  not  know  whether  vapor  in  itself 
has  any  influence  on  the  drying.  Undoubtedly  numerous  in- 
stances of  the  benefits  of  steam  are  to  be  found,  in  high  volume 
air  progressive  dryers,  in  which,  as  will  be  seen  under  the 
discussion  of  progressive  dryers,  saturation  is  not  complete  at 
the  exhaust  end.  In  such  instances  the  steam  introduced  at 
the  hot  end  simply  serves  to  increase  the  humidity  in  such 
parts  of  the  tunnel  where  humidity  is  needed  to  prevent  loss 
through  too  rapid  drying.  We  have  seen  steam  introduced 
into  the  hot  air  entering  the  dryer  in  a  number  of  instances 
with  beneficial  results,  but  we  must  admit  that  we  cannot  see 
what  part  it  plays  in  safer  drying,  unless  it  corrects  a  fault 
in  the  dryer  and  adjusts  the  degree  of  saturation  to  a  safe 
degree. 

Figures  46  and  47  in  plan  and  section  show  the  general 
principles  of  a  hot  air  periodical  dryer  (Bechtel)  which  is 
extensively  used  in  the  United  States  and  Canada. 

There  are  a  series  of  parallel  tunnels  opening  on  top  into 
the  dryer  building.  A  cross  duct  at  one  end  connects  these 
tunnels  with  a  fan  with  steam  coils  attached.  The  inlets  to 
the  floor  tunnels  are  controlled  by  dampers  hinged  at  the  top. 
On  either  side  of  the  open  floor  tunnels  are  stringers  from 
four  to  six  inches  high  above  the  floor  level  which  serve  as 
supports  for  the  pallets  containing  the  bricks.  The  pallets 
are  loaded  at  the  machine  with  eighty  to  one  hundred  and 
twenty  bricks  each  and  carried  to  the  dry  room  on  lifting 
trucks  and  placed  on  the  stringers  over  the  tunnel.  As  soon 
as  a  tunnel  is  fully  covered,  burlap  is  thrown  over  the  bricks, 
and  the  hot  air  is  turned  into  the  tunnel  duct.  The  burlap 
serves  as  a  blanket  to  prevent  rapid  escape  of  the  air  and 
permits  the  latter  to  become  highly  saturated  with  moisture. 

Waste  heat  from  cooling  kilns  may  be  used  in  connection 
with  the  steam  coils,  and  as  the  latter  may  be  heated  during 
the  day  by  exhaust  steam  the  drying  may  be  largely  done 
with  waste  heat. 


DRYING    CLAY    WARES 


It  is  common  practice  to  use  eight  sections  of  coils,  each 
section  to  have  four  rows  of  one-inch  pipes  staggered  to  in- 
sure thorough  contact  with  the  incoming  air.  For  brick  dry- 
ers the  pipes  are  spaced  2%  inches  on  centers  and  the  free 


1 

""* 

r 

i 

^" 

1 

1 

1 
I 
I 

1 

1 

1 

~ 

... 

"~ 

~" 

~" 

1 

1 

1 

1 

1 
1 

— 

_ 

... 

— 

! 

| 

i 

| 

! 

L. 

E 

± 

= 

ii- 

f 

L. 

j 

- 

Fig. 


area  so  obtained  is  approximately  one  and  one-half  times  the 
area  of  the  fan  inlet. 

The  fans  used  are  disc,  plate,  or  squirrel  cage  type,  but 
the  latter  is  the  most  economical  in  power  consumption  and 
the  first  mentioned  the  least  economical. 

Carpenter  has  shown  that  the  economic  limit  of  the  num- 


DRYING    CLAY    WARES 

ber  of  rows  of  pipes  in  a  heater  is  between  16  and  24,  and 
in  consequence  we  will  get  no  return  from  any  pipes  m  ser- 
ies in  excess  of  twenty-four  except  at  air  velocities  which 


would  not  be  considered  in  designing  a  clay  ware  dryer.  This, 
of  course,  applies  to  a  constant  pressure. 

As  stated  above,  the  common  practice  is  to  use  eight  sec- 


DRYING    CLAY    WARES  97 

tions  containing  a  total  of  thirty-two  pipes  but  six  sections 
having  a  total  of  twenty-four  pipes  are  arranged  for  exhaust 
or  low  pressure  steam,  and  the  final  two  sections  are  sup- 
plied with  high  pressure  live  steam.  In  this  way  we  get  the 
value  of  the  large  amount  of  latent  heat  in  the  exhaust  steam 
and  after  the  air  has  reached  approximately  the  temperature 
of  boiling  water,  it  is  brought  in  contact  with  the  high  tem- 
perature live  steam  pipes  and  its  temperature  is  raised  above 
212  degrees  F.  by  the  sensible  heat  of  the  high  pressure. 

Figure  48  is  a  sectional  elevation  of  steam  coils,  waste 
heat  duct  and  fan.  A  damper,  or  set  of  dampers,  is  provided 
so  that  any  proportion  of  the  necessary  volume  of  air  can 
be  obtained  from  the  coils  or  duct.  When  there  is  no  waste 
heat  available  from  the  kiln,  the  duct  to  the  fan  is  closed 
and  all  the  air  is  drawn  through  the  coils  involving  in  many 
instances  the  use  of  live  steam  to  maintain  a  required  air 
temperature.  When  the  waste  heat  has  an  excessive  tem- 
perature it  is  tempered  with  air  through  the  coils  and  oh  such 
occasions  nothing  but  exhaust  (waste)  would  be  used  in  the 
coils.  As  the  kilns  cool  the  volume  of  air  from  them  may  be 
increased,  and,  as  needed,  live  steam  is  introduced  into  the 
coils. 

Figure  49  and  Figure  50  show  in  plan  and  elevation  the 
general  principles  of  a  properly  connected  heater.  Both  high 
pressure  and  exhaust  steam  are  conducted  to  the  coils  and 
controlled  by  valves  and  traps  so  that  we  may  use  exhaust  or 
low  pressure  live  steam  in  six  sections,  and  high  pressure 
steam  in  two  sections,  or  all  the  sections  may  have  exhaust 
steam  or  high  pressure  steam  as  desired,  when  suitable  traps 
are  provided. 

The  condensation  water  may  be  drained  back  to  a  receiv- 
er and  automatically  pumped  back  into  the  boilers,  or  tilting 
traps  may  be  used  to  deliver  the  water  from  the  coils  to  the 
boilers.  Frequently  we  use  a  vacuum  pump  to  exhaust  the 
coils  and  in  this  way  get  greater  efficiency. 

There  are  many  modifications  in  fitting  up  the  equipment, 
and  a  variety  of  traps,  impulse  valves,  etc.,  which  apply  un- 
der different  conditions,  but  it  is  not  the  purpose  of  this  arti- 
cle to  discuss  them.  Every  problem  should  be  worked  out 
by  a  competent  steam  engineer  and  installed  under  his  di- 
rection. 

One  advantage  of  this  type  of  dryer  is  that  there  are  no 
steam  pipes  under  the  bricks  to  be  damaged  and  covered  by 
broken  bricks  falling  from  the  pallets,  and  such  debris  in  the 
tunnels  can  be  easily  cleaned  out  from  time  to  time. 


DRYING    CLAY    WARES 


It  is  very  important  that  the  ducts  be  properly  propor- 
tioned throughout  and  the  openings  from  the  floor  ducts  be 
adjusted  to  insure  a  uniform  pressure  in  all  parts  of  the  sys- 
tem, otherwise  the  bricks  nearest  the  cross  duct  and  those 
over  the  tunnels  nearest  the  fan  will  dry  first.  Since  in  many 
installations  it  is  necessary  to  entirely  unload  a  tunnel  before 
resetting  it,  and,  if  the  hot  air  is  not  properly  distributed,  the 
bricks  most  distant  from  the  fan  will  be  slow  drying,  and 


Fy.49 


I 


ve.3 
i 


\-T  i  o 

r^  TA  K 

further,  since  the  capacity  of  the  dryer  will  be  determined 
by  these  slow  drying  parts  of  the  system,  it  is  highly  import- 
of  the  6t  Pr°Per  diStribution  of  the  hot  air  to  ^1  Parts 


for  ttt  this  dryer'  or  a 

LLh  ™  '<rth°Ut  ConsultinS  the  designers,  usually  lose 

ration     T  ™  f  "^  ^  ^^  in  increased  cost  of  ope- 
ration,    than    they    save    In     the     installation.       The     distri- 


DRYING    CLAY    WARES 


button  of  air  volumes  cannot  be  accurately  pre-determined 
and  the  builder  of  any  equipment  requiring  the  uniform  dis- 
tribution of  air  or  gases  who  has  frequent  opportunity  to 
practically  adjust  the  equipment  to  the  highest  efficiency  can 
work  out  the  problems  of  a  new  installation  much  more  ef- 
fectively than  one  whose  knowledge  of  the  dryer  is  based  on 
the  operation  in  a  neighboring  factory,  and  the  efficiency  of 
a  properly  constructed  equipment  will  be  a  profitable  invest- 
ment. 


,  /n  to.  Ke 


H  O  W   I 


G.     E.X.MAU  £>~T 

e.  SO 


Fief 


This  is  the  first  dryer  we  have  described  in  which  the 
heating  pipes  are  outside  the  dryer,  but  we  will  find  similar 
heating  systems  in  other  types  of  dryers  and  a  brief  discus- 
sion of  this  method  of  heating  will  not  be  out  of  place  here. 

In  sectional  coil  heaters  used  in  connection  with  fan  draft, 
the  coils  are  always  placed  outside  the  dryer  and  the  air  is 
sucked  through  them  and  forced  into  the  dryer. 

One  fact  which  becomes  apparent  is  that  we  need  no  long- 
er consider  the  radiated  heat,  or,  at  least  only  in  such  degree 


100  DRYING    CLAY    WARES 

as  the  casing  and  internal  construction  material  other  than 
the  piping  are  heated  by  radiation  and  in  turn  give  up  the 
heat  by  conduction  to  the  air  passing  through  the  heater.  The 
radiated  heat  which  we  may  neglect  is  not  lost,  however,  ex- 
cept in  part  that  from  the  outside  casing  of  the  coils,  since 
it  passes  from  pipe  to  pipe  and  becomes  useful  in  maintain- 
ing the  temperature  of  the  surface  of  the  pipe,  or,  in  other 
words,  it  reduces  the  condensation  which  otherwise  would  oc- 
cur. In  the  direct  heating  system  we  have  radiated  and  con- 
vected heat  applied  to  the  purpose  of  drying  and  each  re- 
quires its  proportion  of  steam  condensation.  In  the  indirect 
heating,  convection  alone  carries  heat  into  the  dryer  but  the 
radiation  by  maintaining  the  surface  temperature  may  be 
said  to  be  given  up  to  the  dryer  by  convection,  not  to  increase 
the  convected  heat  but  to  reduce  the  condensation  loss.  The 
convected  heat  carried  into  the  dryer  is  constant  for  any  given 
difference  of  temperature  and  fixed  air  velocity  and  we  get 
less  heat  from  the  indirect  method  than  by  the  direct  and  cor- 
respondingly less  condensation,  but  we  can  increase  the  con- 
densation and  thereby  the  convected  heat  by  increasing  the 
velocity  of  the  air.  We  would  not  think  of  putting  the  pipes 
outside  the  dryer  in  a  natural  ventilation  system,  because, 
first,  as  previously  pointed  out,  the  volume  of  air  is  insuffi- 
cient to  carry  the  heat  required,  and  second,  even  though  the 
air  volume  were  sufficient  we  would  have  to  greatly  increase 
the  amount  of  piping.  Increased  air  velocity,  however,  com- 
pletely changes  the  situation. 

There  are  several  formulas  to  determine  the  temperatures 
available  for  steam  coils,  but  they  are  not  convenient  for  the 
clayworkers'  use. 

The  manufacturers  of  heating  and  ventilating  equipment 
publish  the  data  in  the  most  convenient  form,  and  the  follow- 
.  ing  table  is  taken  from  Bulletin  No.  273,  of  the  American 
Blower  Company,  Detroit,  Michigan. 

Table  No.  1. 

To  Determine  Temperature   Rise  for  Any  Steam   Pressure  or 
Initial  Temperature. 

T t  K=constant  as  follows. 

R=  L     l  R=rise. 

K    '  t=temperature  incoming  air. 

T=temperature  steam. 
K  is  as  follows  for  any  Pressure  and  Initial   Temperature: 


DRYING    CLAY    WARES 


101 


No.  of 

Sections 

300' 

600' 

900' 

1,200' 

1,500' 

Deep. 

Vel. 

Vel. 

Vel. 

Vel. 

Vel. 

1 

3.9 

4.91 

5.57 

6.2 

6.66 

2 

3.19 

2.76 

3.13 

3.48 

3.75 

3 

1.615 

2.04 

2.30 

2.56 

2.75 

4 

1.333 

1.68 

1.91 

2.12 

2.28 

5 

1.21 

1.46 

1.66 

1.85 

1.99 

6 

1.142 

1.32 

1.49 

1.66 

1.785 

7 

1.11 

1.24 

1.385 

1.54 

1.66 

8 

1.088 

1.19 

1.310 

1.44 

1.55 

9 

1.072 

1.152 

1.26 

1.36 

1.46 

10 

1.06 

1.130 

1.220 

1.305 

1.40 

If  "t"  is  above  Zero  add  to  "R"  for  Final  Temperature. 

If  "t"  is  below  Zero  deduct  from  "R"  for  Final  Temperature. 


Table  No.  2. 
Properties  of  Saturated  Steam. 


Gage 

Latent 

Total 

Pressure 

B.  T.  U.  in 

B.  T.  U.  in 

B.  T.  U.  in 

inLbs. 

Temp.  F. 

Water 

Steam 

Steam 

0 

212.0 

180 

970.4 

1150.4 

1 

215.3 

183.4 

968.1 

1151.5 

2 

218.5 

186.6 

966.2 

1152.8 

3 

221.4 

189.6 

964.3 

1153.9 

4 

224.4 

192.5 

962.4 

1154.9 

5 

227.2 

195.3 

960.6 

1155.9 

6 

229.8 

198.0 

958.8 

1156.8 

7 

232.3 

200.5 

957.2 

1157.7 

8 

234.8 

203.0 

955.6 

1158.6 

9 

237.1 

205.4 

954.0 

1159.4 

10 

239.4 

207.7 

952.5 

1160.2 

15 

249.7 

218.2 

945.5 

1163.7 

20 

258.8 

227.4 

939.2 

1166.6 

25 

266.8 

235.6 

933.6 

1169.2 

30 

274.0 

243.0 

928.5 

1171.5 

40 

286.7 

255.9 

919.4 

1175.3 

50 

297.6 

267.2 

911.3 

1178.5 

60 

307.3 

277.1 

903.9 

1181.0 

70 

316.0 

286.0 

897.3 

1183.3 

80 

329.9 

294.3 

891.0 

1185.3 

90 

331.1 

301.8 

885.3 

1187.1 

100 

337.8 

309.0 

879.8 

1188.8 

125 

352.2 

324.4 

867.8 

1192.2 

150 

366.3 

338.0 

857.0 

1195.0 

200 

387.8 

361.3 

837.9 

1199.2 

102 


DRYING    CLAY    WARES 


To  determine  the  temperature  rise  we  take  the  difference 
between  the  temperature  of  the  steam  at  any  pressure,  (table 
2)  Ind  the  temperature  of  the  air  and  divide  by  the  proper 
actor  from  table  1,  which  gives  us  the  desired  temperature. 

If  we  have  both  low  and  high  pressure  steam,  we  deter- 
mine the  temperature  for  the  low  pressure  then  with  this 
temperature  as  the  value  of  "t"  we  determine  the  tempera- 
ture from  the  high  pressure  sections. 

For  example   if  we  have  six  sections  on  exhaust  steam  at 
5  pounds  back  pressure,  and  two  sections  at  60  pounds  pres- 
sure  with  the  air  at  60  degrees  and  an  initial  velocity  of  9( 
feet 'per  minute,  we  proceed  as  follows:     Low  pressure 
perature  from  table  2,  227°. 

227—60 

— =112°  F.=gain  in  temperature 
1.49  (from  table  1) 

112  +  60=172°=temperature    after    passing    the    six    low    tem- 
perature coils. 
High  pressure  temperature  from  table  2,  307° 

307—172 

— =43°=additional  gain  in  temperature 
3.13 

172+43=215°=final  temperature. 

It  is  not  claimed  that  these  tables  give  accurate  results, 
but  they  are  sufficiently  close  for  practical  work,  and  they 
have  the  advantage  of  a  ready  reckoning  which  the  busy  man 
desires.  Greater  accuracy  can  be  obtained  by  calculating  the 
velocity  for  each  section  from  the  formula: 
V  x 


491+(t— 32)        491  +  (T— 32) 

and  find  the  factor  for  the  calculated  velocity  by  interpola- 
tion in  table  1.  For  instance,  if  in  the  above  example  we  de- 
termine the  velocity  after  passing  six  sections  we  get  a  ve- 
locity of  1,094  feet  per  minute  and  the  factor  for  two  sections 
will  be  3.35.  This  factor  gives  us  an  additional  temperature 
of  40  degrees  instead  of  43  degrees  as  previously  determined. 
The  error  is  appreciable  and  would  be  still  more  so  if  we  were 
to  calculate  the  velocities  after  passing  each  section,  but  the 
difference  will  be  well  within  the  margin  of  safety  allowed  by 
engineers  especially  in  problems  of  this  character  for  which 
there  is  not  sufficient  experimental  data  upon  which  to  base 
an  accurate  estimate. 

The  problem  of  determining  the  amount  of  piping  required 
for  any  given  requirement  will  be  worked  out  in  the  discus- 


DRYING    CLAY    WARES  103 

sion  of  progressive  dryers  and  need  not  be  taken  up  now, 
except  to  outline  the  data. 

If  the  one  inch  pipes  are  spaced  2%  inches  the  space  be- 
tween will  be  1.435  inches  and  8.37  spaces  one  foot  long  re- 
quiring an  equal  number  of  pipes  will  be  found  in  each  square 
foot  of  free  area.  This  gives  us  8.37-=-3=2.79  square  feet 
heating  surface  in  each  row  per  square  foot  of  free  space,  or 
2.79X32=89.28  square  feet  of  radiating  surface  in  an  eight 
section  heater  for  each  square  foot  of  free  space.  If  the  ve- 
locity is  900  feet  per  minute  we  have  this  many  cubic  feet  of 
air  per  minute  heated  to  a  temperature  of  215  degrees  as  prev- 
iously determined,  by  89.28  square  feet  of  radiating  surface, 
or  reduced  to  pounds  there  will  be  .074X900=66.6  pounds  of 
air  per  minute.  The  mean  specific  heat  of  air  for  volume  is 

[.0188-f. 0000009    (T+t)]    (T— t) 
and  for  weight  is 

[.2344-.000012   (T+t)]    (T— t). 

From  this  we  find  that  we  get  1640  B.  T.  U.  per  hour  per 
square  foot  of  radiating  surface,  which  is  a  decided  advance 
over  the  return  in  B.  T.  U.  in  natural  ventilation.  Having  de- 
termined the  B.  T.  U.  per  foot  of  piping  we  only  need  to  esti- 
mate the  dryer  requirements  to  ascertain  the  quantity  of 
piping  required. 

The  Boss  Dryer. 

Fig.  51  shows  a  recent  dryer  development  (Boss)  in  which, 
the  lifting  car  and  pallet  system  is  used,  but  the  heating  ar- 
rangement is  quite  different  from  any  other  type  of  dryer. 

Between  the  car  tunnels  are  superheating  tunnels  con- 
taining auxiliary  steam  pipes. 

The  main  heating  equipment  is  a  mass  of  steam  pipes  in 
the  main  air  duct  in  front  of  the  fan.  (Not  shown  in  the 
sketch.)  In  this  dryer,  the  steam  pipes  are  between  the  dry- 
er and  fan.  Exhaust  steam  exclusively  is  used  in  the  piping 
in  the  main  duct.  The  air  entering  through  the  fan  is  forced 
among  the  pipes  in  the  main  duct  and  becomes  heated  to  the 
full  possibility  of  exhaust  steam.  Exhaust  steam  is  used  in 
the  heating  tunnels  of  the  dryer  proper  and  the  air  in  pass- 
ing these  pipes  is  brought  up  to  the  full  temperature  permis- 
sible from  exhaust  steam.  Were  it  desired,  live  steam  in  the 
auxiliary  (tunnel)  piping  would  produce  a  higher  temperature. 

I'uder  the  szeam  pipe  tunnels  are  air  ducts  with  inlets  into 
the  heating  tunnels,  which  connect  at  the  receiving  end  of 
the  dryer  with  a  cress  duct  from  a  fan. 


104 


DRYING    CLAY    WARES 


The  pallets  have  double  floors,  the  bottom  being  tight 
and  the  top  perforated,  in  other  words,  they  are  flat  boxes 
with  perforated  tops.  When  the  pallets  are  in  place  they 
form  a  part  of  the  hot  air  distribution  system,  since  they 
cover  ducts  from  the  heating  tunnels  and  have  openings 
through  their  bottoms  to  admit  the  hot  air  into  the  palle 
and  thence  the  air  rises  through  the  mass  of  bricks  and 
escapes  at  the  top.  To  prevent  lateral  escape  of  the  air  mov- 


(Section 


able  curtains  are  adjusted  to  the  sides  after  the  tunnels  are 
fully  covered  with  loaded  pallets. 

Waste  heat  or  any  heat  source  is  applicable  to  this  sys- 
tem, in  conjunction  with  the  steam  heating  or  independent 
thereof. 

The  drying  principle  is  exclusively  up  draft,  which  has 
long  been  held  by  brick  makers  to  be  the  best  method  of 
drying.  The  upward  passage  of  the  air  among  the  bricks  is 


DRYING    CLAY    WARES 


105 


retarded  to  any  desired  practical  degree  by  close  setting  of 
the  bricks  and  the  top  courses  may  be  set  to  act  as  dampers, 
thus  preventing  too  rapid  escape  of  the  air. 

There  are  no  pipes  in  the  car  tunnels  and  the  heating  tun- 
nels are  covered  so  that  no  debris  can  get  into  the  heating 
tunnels  except  such  as  may  fall  through  the  small  connection 
between  pallet  and  heating  tunnels  when  the  pallets  are  re- 
moved. 

The  Pipe  Rack  Dryer. 

The  pipe  rack  dryer  shown  in  Fig.  52  is  used  in  many 
yards  where  the  clay  will  stand  such  severe  treatment.  It 
is  so  well  known  to  the  clayworking  fraternity  that  a  de- 
scription is  hardly  necessary. 

The  dryer  is  simply  a  series  of  steam  pipe  racks  upon 


/=•/>.  52 


which  the  flat  steel  pallets  containing  the  bricks  are  placed. 

It  is  customary  to  build  the  racks  in  four  sections  each 
holding  7,500  to  10,000,  making  the  daily  capacity  of  each 
unit  dryer  30,000  to  40,000  bricks. 

The  pallets  loaded  with  bricks  are  delivered  into  the  dry- 
er in  the  passageway  between  the  racks  on  rope  conveyors 
and  the  empty  pallets  are  returned  on  the  same  conveyor 
using  the  under  return  ropes  for  the  empty  pallets. 

A  thirty  thousand  capacity  dryer  will  have  four  quarters 
each  containing  eight  sections  ten  feet  long  and  fourteen 
rows  high.  In  each  row  there  are  five  one-inch  pipes.  The 
total  piping  therefore  for  30,000  bricks  per  day  is  22,400  lineal 
feet,  not  including  connections  and  fittings.  Reducing  this 
to  square  feet  of  heating  surface  per  thousand  bricks  we 


DRYING    CLAY    WARES    .  107 

get  249,  which  is  greatly  in  excess  of  the  piping  requirement 
as  figured  for  periodical  tunnels. 

It  is  likely  that  the  degree  of  saturation  is  not  high,  but 
the  dryer  can  be  depended  upon  to  deliver  its  full  quota  of 
bricks  every  24  hours,  which  Is  a  very  important  factor  in 
economical  operations. 

Tender  Clay  Dryer. 

Figs.  52  and  53  show  in  plan  and  section  one-half  of  a  re- 
cently patented  periodical  dryer,  the  purpose  of  which  is  con- 
trol of  the  drying  conditions  in  each  tunnel  independent  of 
the  other  tunnels.  It  has  a  series  of  double  tracked  tunnels, 
"A,"  as  in  some  other  types  of  tunnel  dryers  and  under  each 
track  is  a  distributing  duct,  "B,"  for  the  hot  air,  with  grad- 
uated inlets,  "C,"  through  the  tunnel  floor,  similar  to  the  dis- 
tributing ducts  in  waste  heat  progressive  dryers.  The  moist- 
ure ladened  air,  after  leaving  the  ware,  escapes  through  a 
series  of  ventilators,  "D,"  in  the  roof  of  the  tunnels. 

There  are  two  main  cross  ducts — "G"  and  "E" — one  above 
the  other,  to  supply  the  distributing  ducts  with  air.  From 
each  cross  duct  there  are  damper  controlled  openings,  "I"  and 
*J,"  into  a  series  of  mixing  chambers,  "K,"  directly  connected 
with  the  distributing  ducts. 

The  lower  cross  duct,  "E,"  connects  with  a  fan,  "F,"  and 
the  air  supply  is  direct  from  the  outside;  namely,  cold  air. 

The  upper  cross  duct,  "G,"  has  a  separate  fan,  "H,"  draw- 
ing heated  air  from  any  source  of  heat — kiln,  auxiliary  fur- 
naces, heating  coils,  etc. 

The  illustrations  show  the  cross  ducts  in  the  center  of 
the  dryer,  but  this  is  not  essential.  It,  however,  enables  the 
builder  to  construct  a  dryer  twice  as  long  as  one  having  the 
heat  and  air  supply  at  one  end,  or  in  a  shorter  dryer  it  in- 
sures more  perfect  distribution  of  the  heat  and  air  and  in 
consequence  a  more  equitable  temperature  from  end  to  end 
of  the  dryer,  and  more  uniform  drying  of  the  ware. 

Cold  air  alone,  or  hot  air,  or  a  mixture  of  cold  and  hot  air 
in  any  degree  and  in  any  volume,  may  be  forced  into  each, 
any,  or  all  tunnels,  as  desired. 

'  After  a  tunnel  is  filled  with  ware,  the  damper  inlet,  "I," 
from  the  cold  air  cross  duct  may  be  slightly  opened,  thus  per- 
mitting a  small  volume  of  low  temperature  air  to  enter.  As 
the  ware  will  stand  more  rapid  circulation,  the  cold  damper 
can  be  opened  to  greater  extent.  Following  this  the  tern- 


108  DRYING    CLAY    WARES 

perature  of  the  air  can  be  increased  by  opening  the  inlet,  "J," 
from  the  hot  air  cross  duct  and  can  be  carried  higher  and 
higher  by  further  increase  of  the  latter  opening,  at  the  same 
time  reducing  the  cold  air  inlet. 

No.  1  tunnel  may  be  filling  and  the  air  inlets  will  be 
closed,  thus  disconnecting  this  tunnel  from  the  cross  air 
ducts.  No.  2  tunnel  may  be  unloading  and  its  dampers  also 
will  be  closed.  The  drying  in  No.  3  may  be  so  far  advanced 
that  the  full  hot  air  temperature  may  be  used  and  it  may 
get  its  supply  of  air  entirely  from  the  hot  air  duct,  while  No. 
4  tunnel  may  require  a  mixture  of  hot  and  cold,  in  which 
case  both  hot  and  cold  air  inlets  will  have  their  dampers  par- 
tially opened,  and  No.  5  tunnel,  just  starting,  may,  perhaps, 
safely  use  only  cold  air.  It  may  be  that  a  factory  is  making 
two  kinds  of  ware,  one  of  which  requires  a  slow  careful  dry- 
ing, while  the  other  will  stand  rapid  treatment.  It  is  claimed 
that  all  of  these  conditions  are  practicable  in  this  dryer. 

At  first  glance  one  would  say  that  a  progressive  regula- 
tion would  be  difficult  to  control,  but  regular  progression 
probably  is  not  the  intention  of  the  inventor,  nor  is  it  neces- 
sary. The  operation  would  likely  be  in  stages — two  or  at 
most  three — and  experiment  must  determine  the  temperature 
and  duration  of  each  stage  for  the  different  wares.  Each 
tunnel  is  provided  with  a  recording  thermometer,  and  when  a 
tunnel  is  first  connected,  the  dampers  may  be  adjusted  to  the 
desired  temperature  for  the  first  stage.  At  the  expiration  of 
the  first  period  the  dampers  may  be  adjusted  to  bring  the 
temperature  to  that  required  during  the  second  period,  and 
again  for  the  third  period,  should  a  third  period  be  required. 
Intermediate  adjustments  would  be  required  to  correct  varia- 
tions in  the  temperature  of  the  air  from  the  sources  of  supply. 

The  dryer  is  essentially  for  tender  drying  clays,  such  as 
require  careful  treatment  in  the  start  but  which  may  be  fin- 
ished rapidly.  The  ordinary  waste  heat  dryer  has  to  be 
adjusted  for  the  careful  treatment  all  through  in  order  to 
protect  the  tunnels  freshly  filled  and  in  consequence  the  full 
drying  period  is  excessive,  often  times  impractically  so.  For 
such  clays  a  dryer  which  enables  the  operator  to  advance 
the  rate  of  drying  in  each  tunnel  independently,  has  a  decid- 
ed advantage.  For  clays  which  will  stand  abuse  in  drying, 
the  temperature  can  be  maintained  at  a  maximum  all  the 
time  and  a  single  control  through  the  hot  air  fan  would  be 
simpler  than  any  multiple  control.  This  single  control  is 


DRYING    CLAY    WARES  109 

equally  applicable  to  the  above  described  dryer  in  which 
event  the  cold  air  fan  and  duct  become  superfluous. 

A  feature  of  the  dryer,  especially  for  hollow  ware,  is  that 
the  movement  of  the  air  is  directly  upward  through  the  ware 
and  the  drying  is  more  uniform  and  there  is  less  damage  in 
in  consequence.  There  are  several  dryers,  both  periodic  and 
progressive,  in  use,  having  this  updraft  feature,  and  the  pro- 
moters of  the  horizontal  draft  types  justly  claim  that  one  can- 
not get  full  value  from  the  heat  by  a  single  short  passage  of 
the  air  through  the  ware — in  other  words,  that  the  satura- 
tion will  not  be  complete  and  there  will  be  a  waste  of  heat. 
This  is  true,  but  of  no  moment,  if  only  waste  heat  is  used  in 
the  drying-  If  heat  is  generated  for  the  drying,  naturally  the 
less  degree  of  saturation  will  require  more  fuel,  but  this  may 
be  offset  by  quicker  and  safer  drying. 

A  standardization  of  sixty-eight  Ohio  clays  showed  15  per 
cent  that  could  be  safely  dried  in  commercial  operations  in 
twenty-four  hours;  51  per  cent  required  from  twenty-four  to 
seventy-two  hours;  19  per  cent  were  only  safe  between  seven- 
ty-two hours  and  seven  days,  while  the  remainder  required  in 
excess  of  seven  days.  Ohio  is  a  favored  state  in  its  clay- 
working  materials,  yet  the  percentage  of  clays  that  rank  first 
class  in  drying  behavior  is  low.  There  are  few  states  that  can 
make  as  good  a  showing  as  Ohio,  and  there  are  some  states 
in  which  good  drying  clays  are  so  rare  that  2  per  cent  will 
probably  include  all  that  are  first  class  in  drying  qualities. 
There  is,  therefore,  a  large  field  for  a  tender  clay  dryer. 


110  DRYING    CLAY    WARES 


CHAPTER  X. 
Pottery  Drying. 

UNDER  THE  HEAD  of  pottery  we  include  all  ware 
which  is  the  work  of  the  potter— white  and  yellow 
ware,  porcelain,  electrical  ware,  sanitary  products, 
stoneware,  etc. 

The  drying  of  these  wares  requires  different  conditions, 
depending  upon  the  size  and  shape  of  the  ware,  upon  the 
mixture  and  upon  the  process  of  manufacture. 

In  view  of  the  varying  treatment  required  in  the  several 
wares,  there  is  no  general  type  of  dryer  applicable  to  all, 
and  it  is  beyond  the  province  of  this  article  to  describe  the 
drying  methods  in  any  detail. 

Pottery  is  largely  the  product  of  hand  labor  and  each  piece 
as  it  leaves  the  potter's  hands,  or  at  most  several  pieces  on 
a  pallet,  is  taken  to  the  dry  room  by  hand. 

Since  hand  work  enters  so  largely  into  the  manufacture 
of  pottery,  it  is  essential  that  the  work  rooms  have  abundance 
of  light  and  air,  and  in  general  we  find  the  potters'  benches, 
jollys,  jigs,  etc.,  along  the  outer  walls  of  the  factory  building. 

Since  the  ware  is  moved  by  hand  another  essential  fea- 
ture is  that  the  distance  from  the  benches  to  the  dry  rooms 
shall  be  a  minimum. 

In  consequence  of  these  two  factors  we  frequently  find  the 
dry  rooms  in  the  center  of  the  manufacturing  room.  The 
distance  to  the  dry  room  is  thus  very  short,  and,  of  course, 
equally  short  from  the  dryer  to  the  finisher  or  back  to  the 
potter,  to  whom  the  molds  must  be  returned. 

A  modern  arrangement  for  some  lines  of  ware  is  the  con- 
tinuous operation  plan,  now  applied  to  so  many  industries,  in 
which  the  raw  materials  enter  at  one  end  and  in  each  stage 
of  the  manufacture  advance  toward  the  warehouse  for  fin- 
ished ware,  which  places  the  dry  room  adjacent  to  the  manu- 
facturing room  and  opening  into  it.  The  succeeding  rooms 


DRYING    CLAY    WARES  111 


depend  upon  the  character  of  the  ware  and  the  process  of 
manufacture. 

The  dry  rooms  for  small  ware  consist  of  a  series  of  com- 
partments four  or  more  feet  wide,  depending  upon  the  size 
of  the  ware,  and  ten  or  more  feet  long,  depending  upon  the 
width  of  the  building  and  the  space  required  for  the  potters. 
On  either  side  of  each  compartment  are  shelves  spaced  to 
suit  the  ware  in  question  and  extending  to  the  ceiling  of  the 
room.  The  passageway  between  the  shelves  is  simply  wide 
enough  for  the  workmen  who  fill  and  empty  the  shelves,  and 
is  made  as  narrow  as  possible  in  order  to  get  a  maximum 
drying  capacity  within  the  allotted  space.  In  some  lines  of 
ware  the  compartments  or  dry  rooms  are  provided  with  doors 
which  close  the  room  when  not  being  filled  or  emptied,  but 
in  other  lines  the  rooms  are  merely  racks,  with  passageways 
for  the  workmen  distributing  the  ware. 

The  heating  is  by  steam  pipes,  usually  three  one-inch 
pipes  along  the  floor  under  each  set  of  shelves,  or  six  pipes 
to  each  room.  As  the  ventilation  is  natural,  usually  very 
crude  and  slow,  the  air  within  the  room  is  practically  heated 
by  radiation,  or,  more  properly  should  we  say,  that  the  fix- 
tures, walls,  shelves,  molds,  etc.,  are  heated  by  radiation, 
and  the  air  is  heated  by  contact  with  the  pipes,  and  with  the 
fixtures,  etc.  In  some  lines  of  ware  which  will  stand  more 
rapid  treatment,  the  pipes  are  distributed  under  each  shelf, 
or  each  alternate  shelf,  which  brings  the  heating  surface  in 
closer  touch  with  the  ware. 

As  in  other  clay  industries,  insufficient  drying  room  is  one 
of  the  handicaps  of  the  pottery  industry,  and  potters  are  con- 
stantly studying  the  problem  of  how  to  increase  the  drying 
operation  within  the  space  available,  without  damage  to  the 
ware  and  without  increased  cost. 

Instead  of  the  rooms  with  passageways  the  dryer  becomes 
more  compact — in  fact,  the  space  occupied  is  more  than 
doubled  in  capacity — by  having  the  shelves  hung  on  trolleys 
and  overhead  tracking,  which  permit  pulling  them  out  for 
filling  and  emptying.  Thus,  each  shelf  is  brought  nearer  the 
potter  and  the  distance  traveled  per  day  by  the  off-bearers 
is  lessened.  Every  foot  increase  in  the  distance  the  ware 
has  to  be  moved  adds  to  the  cost — otherwise  there  would  be 
no  question  in  regard  to  dryer  capacity.  At  first  glance,  the 
movable  shelf  plan  seems  the  most  advantageous,  but  it 
must  be  remembered  that  there  must  be  a  wide  space  be- 


112 


DRYING    CLAY    WARES 


tween  the  potters'  benches  and  the  shelves  to  provide  room 
for  the  shelves  when  pulled  out,  besides  working  room  around 
them. 

It  is  a  question  whether  the  movable  shelves  with  their 
greater  initial  cost  and  greater  cost  of  upkeep  have  any  ad- 
vantage over  the  fixed  racks  with  passageways. 

Mr.  Herford  Hope,  in  a  paper  before  the  sixteenth  annual 
meeting  of  the  American  Ceramic  Society,  and  later  in  a  lec- 
ture before  the  potters  of  Ohio  in  the  Ohio  State  University, 
presented  a  new  type  of  pottery  dryer  adapted  to  small  ware. 


Instead  of  rectangular  rooms  with  shelves  on  either  side,  or 
rectangular  racks  which  can  be  moved  in  and  out  on  trolleys, 
he  suggests  a  hexagonal  closet.  In  the  center  of  each  closet 
there  is  a  vertical  shaft  to  which  the  shelves  are  attached 
radially,  and  the  whole  shelf  contrivance  can  readily  be  swung 
around  a  circle,  similar  to  a  revolving  clothes  horse.  The 
door  to  this  closet  is  opposite  the  potter's  bench,  and  the 
passage  into  it  between  any  two  sets  of  shelves,  is  "V" 
shaped.  As  the  shelves  of  each  section  or  "V"  of  the  shelves 
are  filled,  the  apparatus  is  moved  one  section.  Mr.  Hope 


DRYING    CLAY    WARES 


113 


shows  that  no  greater  floor  space  is  occupied  and  that  the 
off-bearers  travel  less  distance  per  day.  If  no  greater  floor 
space  is  occupied,  and  the  distance  traveled  by  the  off- 
bearers  is  reduced,  the  hexagonal  closet  is  an  advance  over 
the  rectangular,  provided  the  first  cost  and  the  subsequent 
upkeep  are  not  excessive. 

A  vertical  shaft  properly  installed  offers  no  mechanical 
difficulties,  and  the  upkeep  will  be  practically  nothing.  The 
initial  cost  will  be  greater  than  the  fixed  shelves,  but  not  as 
great  as  the  shelves  attached  to  trolleys.  It  seems,  there- 


FCy.  55   Ena  Eicvatfon 

fore,  that  Mr.  Hope  offers  to  the  pottery  industry  a  decided 
advance  in  the  type  and  arrangement  of  the  dry  rooms. 

Prof.  Carl  B.  Harrop,  of  the  Ohio  State  University  School 
of  Ceramics,  in  a  lecture  before  the  potters  of  Ohio,  on  the 
subject  of  "Pottery  Closet  Dryer  Calculations,"  assumes  the 
radiation  and  convection  from  the  steam  piping  to  be  3.25  B. 
T.  U.  per  hour  per  square  foot  of  radiating  surface  per  1 
degree  difference  in  temperature.  In  a  note  he  explains:  "In 
a  case  of  this  kind,  it  is  exceedingly  difficult  to  determine 
just  how  much  of  the  heat  is  radiated  from  the  pipes  and 


114 


DRYING    CLAY    WARES 


how  much  is  carried  away  from  the  pipes  by  convection  (by 
^incoming  air).     Experiments  and  calculations  show  tha 
the  radiation  will  be  approximately  1.25  B.  T.  U. 
erence    2  B    T    U.   (3.25-1.25),  cannot  be  carried  in  by  the 
amount  of  air  at  120  degrees,  which  was  calculated  as  suffi- 
cient to  carry  out  the  moisture. 

"The  explanation  of  this  is  that  there  is  an  internal  cir- 
culation of  the  air  going  on  continuously  inside  the  dryer, 
i  e  some  of  the  air  which  goes  into  the  dryer,  becomes 
heated  rises,  gives  some  heat  to  the  ware,  takes  up  some 
moisture  from  the  air  and  escapes.  Other  portions  of  the 
entering  air  does  not  escape  immediately  but  cools  suffi- 
ciently to  drop  to  the  bottom  of  the  dryer  and  again  passes 
around  the  pipes,  is  reheated  and  rises  again  to  perform 
more  work." 

This  accords  fully  with  our  view  of  the  action  of  a  natural 


Fiq.56     <S ide  £:.  leva  t ion 

draft  steam  pipe  dryer  as  expressed  in  a  previous  article  of 
this  series. 

Prof.  Harrop's  data  is  based  upon  5  pounds  steam  pressure 
and  60  degrees  temperature  of  incoming  air,  and  his  calcula- 
tions develop  the  preceding  diagram — Fig.  54 — as  the  heat  bal- 
ance for  a  white  ware  pottery  dryer. 

The  calculations  lead  to  the  conclusion  that  a  closet  4  feet 
3  inches  wide  (containing  shelves  on  either  side  and  passage- 
way in  the  center),  8  feet  high  and  10  feet  long,  will  require 
six  1-inch  pipes,  10  feet  long,  which  is  common  pottery  prac- 
tice. 

Figs.  55  and  56  show  end  and  side  elevation  of  the  general 
construction  of  fixed  pottery  shelves  for  small  white  ware, 
stoneware,  and  other  ware  not  too  large  to  be  placed  in 
shelves,  and  which  will  not  permit  piping  under  each  shelf. 


DRYING    CLAY    WARES  115 


CHAPTER  XI. 
Terra  Cotta  and  Other  Special  Dryers. 

IT  IS  A  QUESTION  whether  we  are  justified  in  describing 
a  dryer  as  typical  of  terra  cotta.  We  have  seen  terra  cotta 
modeled  in  a  single  piece  over  40  feet  long  and  10  feet 
high.  After  the  modeling  the  piece  in  question  was  cut  into 
sections  to  be  dried  and  burned,  but  each  section  was  so  com- 
plicated that  the  drying  had  to  be  watched  and  controlled.  For 
such  ware,  and  there  is  a  great  deal  of  it,  a  dryer  of  any  type 
is  obviously  out  of  the  question. 

Monumental  pieces  of  terra  cotta  can  only  be  dried  on  the 
modeling  floor,  and  the  drying  process  involves  the  use  of  wet 
cloths  here  and  there  on  each  piece  to  insure  uniform  drying 
without  damage. 

This  is  equally  true  of  other  delicate  wares,  such  as  glass 
pots,  flattening  stones  and  retorts,  some  of  which  exceed  a  ton 
in  weight,  and  the  manufacture  of  which  from  start  to  finish 
requires  several  months. 

Smaller  terra  cotta  products,  such  as  moldings,  etc.,  which 
can  be  cut  into  small  pieces  of  uniform  size,  can  be  dried  more 
quickly,  and  such  pieces  may  be  and  usually  are  removed  from 
the  modeling  floor  to  some  type  of  dryer. 

One  prominent  terra  cotta  plant  has  the  modeling  floor 
(press  room)  underlaid  by  flues  from  the  kilns,  and  all  the 
heat  from  the  burning  kilns  passes  under  this  floor.  At  night, 
arrangement  is  made  to  force  into  the  press  room  hot  air  from 
cooling  kilns.  This  method  is  economical,  but  lacks  sufficient 
control. 

In  two,  perhaps  more,  installations  the  press  room  is  under- 
laid by  steam  pipes  in  sections,  each  section  being  controlled 
by  valves. 

In  several  instances  special  dryers  are  provided  for  ware 
which  will  stand  more  rapid  treatment  than  that  of  the  press 
room. 

Fig.  57  shows  a  dryer  used  in  the  manufacture  of  terra 


116 


DRYING    CLAY    WARES 


: 


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XI 

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DRYING    CLAY    WARES  117 

cotta.  It  is  of  the  tunnel  type  and  two-storied.  The  lower 
tunnel  receives  the  terra  cotta  on  cars.  In  the  upper  tunnel  is 
a  fan  and  set  of  steam  coils. 

The  fan  forces  the  air  through  the  heating  coils,  thence  the 
length  of  the  upper  tunnel,  from  which  it  passes  into  the  lower 
tunnel  and  returns  to  the  fan.  With  no  outlet  for  the  air 
there  will  be  no  incoming  air,  and  the  fan  merely  creates 
circulation.  When  the  air  becomes  fully  saturated  no  drying 
can  take  place. 

In  one  instance  the  practice  was  first  to  continue  this  cir- 
culation until  the  ware  became  fully  heated  up,  then  to  open  a 
door,  "A,"  in  the  end  of  the  upper  tunnel  leading  to  the  fan, 
also  open  the  exit  door,  "D,"  of  the  lower  tunnel,  and  close  the 
opening,  "C,"  between  the  upper  and  lower  tunnels  at  the  fan 
end.  With  these  changes  the  fan  would  draw  air  from  the  out- 
side, force  it  through  the  coils  and  to  the  end  of  the  upper  tun- 
nel, then  down  into  the  lower  tunnel  and  returning  to  exhaust 
at  the  delivery  end  of  the  latter. 

In  two  other  installations  a  small  relief  or  escape  hole, 
"B,"  is  placed  at  the  end  of  the  upper  tunnel  opposite  to  the 
fan.  Part  of  the  air,  forced  through  the  coils  by  the  fan,  es- 
capes through  this  relief  hole,  while  the  remainder  passes 
through  the  ware  and  returns  to  the  fan. 

The  quantity  of  air  which  escapes  through  the  relief  hole 
must  be  replaced  by  outside  air  through  "A."  The  lower  tun- 
nel doors  are  kept  closed  all  the  time  except  in  filling  and  emp- 
tying, and  after  the  inlet  and  vents  are  adjusted  to  the  ware 
no  changes  are  necessary. 

In  this  dryer  we  have  the  most  satisfactory  conditions  for 
drying  difficult  ware — a  rapid  circulation  of  hot,  nearly  satu- 
rated air. 

In  two  factories  which  have  come  under  our  observation 
all  ware  that  has  reached  the  stage  of  rapid  drying  and  can  be 
removed  from  the  press  rooms  (which  have  steam-heated 
floors)  is  taken  to  drying  rooms  which  have  slotted  floors  and 
through  which  air  is  forced  by  a  plate  fan.  The  air  is  drawn 
from  the  outside  through  heating  coils,  or  from  cooling  kilns. 
It  is  also  provided  that  hot  air,  after  passing  through  the  dry- 
ers, may  be  forced  into  the  press  room  to  maintain  the  tem- 
perature of  the  latter  and  provide  a  nearly  saturated  air,  which 
is  desired  for  the  initial  drying  stages  of  freshly  molded 
wares,  and  also  during  the  modeling. 

Typical  waste  heat  progressive  dryers  are  also  used  in 
some  factories. 


118  DRYING    CLAY    WARES 


The  Scott  System. 

The  Scott  system  is  a  method  of  drying,  rather  than  a 
dryer,  since  the  purpose  of  the  system  is  to  eliminate  the 
dryer.  It  is  applicable  to  stiff-mud  bricks.  Dryer  cars,  trucks, 
etc.,  are  replaced  by  conveyor  belts. 

The  take-off  belt  from  the  cutter  is  extended  along  the 
fronts  or  ends  of  the  kilns.  Opposite  the  entrance  of  each  kiln 
a  cross-conveyor  takes  the  bricks  from  the  main  conveyor  into 
and  through  the  kiln,  and  from  this  belt  they  are  taken  off  by 
the  setters. 

The  bricks  are  set  six  to  eight  courses  high,  or  to  whatever 
height  they  will  carry  the  weight.  The  floor  of  the  kiln  is  com- 
pletely covered  to  this  height,  and  it  is  the  aim  of  the  system 
to  have  the  kilns  of  such  size  that  they  will  hold  one  day's  run 
of  machine. 

The  kilns  have  a  system  of  under-floor  flues,  and  after  the 
kiln  floor  is  fully  covered  with  the  first  setting  of  bricks,  hot 
air  by  means  of  a  fan  is  forced  through  these  distributing  flues 
and  up  through  a  perforated  kiln  floor,  among  the  bricks,  and 
escapes  from  the  top  of  the  setting. 

If  the  clay  will  stand  rapid  drying,  the  bricks  so  set  can  be 
dried  during  the  night  sufficiently  to  carry  the  weight  of  a  sec- 
ond setting  on  top  of  the  first  lot. 

In  this  case  the  cross-conveyor  is  raised  to  the  proper 
height  for  a  second  setting,  and  the  total  height  at  the  end  of 
the  second  day  is  twelve  or  more  courses.  At  night  the  heat  is 
again  turned  in,  perhaps  completing  the  drying  of  the  first  set- 
ting, and  hardening  the  second  setting  sufficiently  to  carry  the 
weight  of  a  third  setting.  The  operation  is  repeated  until  the 
kiln  is  filled  to  the  proper  height,  and  the  burning  begins  with 
the  advantage  that,  except  the  last  setting,  the  bricks  are  dry, 
or  nearly  so,  and  heated  up. 

If  the  clays  will  not  stand  such  rapid  work,  two  kilns  are 
used  alternately,  thus  giving  thirty-six  hours  for  drying  each 
setting. 

The  take-offs,  dryer  transfer  men  and  tossers  in  the  kiln 
are  eliminated,  except  one  man  is  required  to  remove  the  waste 
cut  from  the  belt  where  cutters  making  a  waste  cut  are  used, 
and  one  man  is  required  to  transfer  the  bricks  from  the  main 
belt  to  the  cross-conveyor. 

Before  introducing  the  hot-air  system  of  drying,  it  was  at- 
tempted simply  to  set  the  bricks  in  the  kiln  as  high  as  they 


DRYING    CLAY    WARES  119 

would  stand  and  dry  them  during  the  night  by  means  of  light 
fires  in  the  furnaces,  but  this  method  of  drying  failed  because 
it  was  impossible  to  properly  distribute  the  heat,  and  the  sys- 
tem only  became  practical  with  the  introduction  of  hot-air  dry- 
ing as  worked  out  by  Mr.  Scott. 

In  one  instance,  where  natural  gas  was  available,  gas  pipes 
with  a  series  of  jet  flames  were  placed  through  the  arches,  and 
at  night  these  were  kept  burning  to  provide  heat  for  drying. 
We  do  not  know  how  successful  this  gas  firing  method  proved 
to  be,  but  it  has  not  been  duplicated  so  far  as  we  know. 

The  gas-pipe  method  of  drying  is  very  simple,  but  it  is  only 
applicable  to  up-draft  kilns,  while  the  Scott  system  in  its  en- 
tirety is  equally  applicable  to  up-draft,  rectangular  down-draft, 

and  chambered  continuous  kilns,  and  is  in  operation  in  yards 
equipped  with  these  several  types  of  kilns. 

The  Underwood  system  of  burning  up-draft  kilns  with  pro- 
ducer gas  woAild  give  a  wider  use  of  the  Scott  system  of  con- 
veying and  drying,  provided  it  is  practical  to  dry  the  bricks  in 
kilns  with  a  direct  flame,  as  in  the  natural  gas  flame  method 
mentioned  above. 

Among  brickmakers,  especially  those  using  continuous 
kilns,  there  has  been  some  discussion  in  regard  to  the  possi- 
bility of  drying  the  bricks  regularly  set  in  the  kiln,  thus  elimi- 
nating the  dryer. 

The  practicability  of  this  is  doubtful.  In  the  first  place, 
few  clays  will  In  the  green  state  support  the  weight  of  regular 
setting. 

Next  arises  the  difficulty  of  regulating  the  rate  of  burn- 
ing to  that  of  drying,  and  this  has  been  found  a  difficult  prob- 
lem, and  an  unsolved  one  in  this  country,  by  those  who  are 
trying  to  introduce  car  tunnel  kilns  for  drying  and  burning, 
in  which  the  bricks  are  set  on  cars  at  the  machine  and  the 
loaded  cars  travel  successively  through  the  several  stages  of 
drying,  burning  and  cooling  in  a  tunnel  kiln.  The  discussion 
of  this  operation  belongs  under  the  head  of  progressive  dryers 
and  will  there  be  considered. 

However,  if  drying  in  the  continuous  kiln  becomes  prac- 
ticable, it  will  lead  to  a  decided  change  in  the  construction  of 
our  kilns,  and  in  this  event  a  kiln  along  the  lines  of  the  Ger- 
man zigzag  kiln,  with  its  low  crowns  and  convenience  for  me- 
chanical setting,  would  likely  be  developed. 

Among  the  economies  suggested  by  those  discussing  the 
problem  is  that  of  fuel,  the  claim  being  made  that  the  drying 


120  DRYING    CLAY    WARES 

could  be  done  entirely  with  waste  heat.  This  is  a  mistake. 
The  only  waste  heat  in  a  continuous  kiln  is  that  which  escapes 
through  the  stack  or  draft  fan,  which  is  very  little,  and  that 
lost  by  radiation,  which  is  not  recoverable. 

If  we  succeed  in  Utilizing  the  continuous  kiln  for  drying  we 
must  increase  the  present  burning  fuel  consumption  from  50 
per  cent,  to  100  per  cent,  which  in  itself  is  a  problem  in  many 
continuous  kilns  at  present  operated.  In  our  present  knowl- 
edge, and  with  the  present  continuous  kiln  development,  dry- 
ing stiff-mud  products  in  the  kiln  is  not  feasible,  except  possi- 
bly in  some  type  of  car  tunnel  kiln,  of  which  there  are  now  one 
or  two  promising  prospects  being  worked  out. 

Car  Tunnel  Kilns. 

A  car  tunnel  kiln  may  combine  the  processes  of  drying  and 
burning  in  one  operation. 

The  car  tunnel  kiln  dates  back  more  than  one  hundred  and 
sixty  years.  The  first  patent  is  seventy-five  years  old,  while 
the  more  or  less  successful  modern  kiln  was  patented  more 
than  forty  years  ago. 

Several  attempts  have  been  made  to  introduce  the  kiln  in 
this  country  with  very  meagre  results  in  drying. 

The  principle  of  the  operation  is  that  the  green  ware  on 
cars  from  the  machine  enters  the  tunnel  at  one  end  and  passes 
successively  through  drying,  watersmoking,  heating  up,  burn- 
ing and  cooling  zones,  and  arrives  at  the  exit  end  finished  and 
ready  for  shipment. 

It  has  been  found  impractical  in  general  to  adjust  the  ope- 
ration of  the  kiln  to  suitable  drying  conditions,  and  the  gen- 
eral opinion  in  countries  where  the  kiln  has  reached  its  high- 
est development  is  that  the  drying  must  be  carried  on  in  a 
separate  compartment.  Undoubtedly,  some  shales  and  clays 
in  this  country  will  stand  the  severe  drying  condition  neces- 
sarily developed  in  the  tunnel  kiln  in  order  to  keep  pace  with 
the  rapid  burning,  but  for  general  application  the  combined 
operation  in  a  unit  tunnel  is  not  practical  in  the  present  com- 
mercial development  of  the  kiln. 

The  practicability  of  the  single  tunnel  may  be  questioned 
for  any  clay,  or  at  least  except  in  rare  instances,  aside  from 
the  ability  of  the  clay  to  stand  severe  drying  treatment. 

There  are  many  clays  that  will  stand  any  kind  of  abuse  in 
drying,  but  there  is  the  question  of  heat  and  moisture  to  be 
considered.  Clays  contain  combined  water  up  to  14  per  cent 


DRYING    CLAY    WARES  121 

of  their  weight,  and  in  the  tunnel  kiln  we  have  the  vapor  of 
this  combined  water  to  contend  with  as  well  as  the  moisture 
vapor  which  is  removed  in  a  dryer. 

In  the  car  tunnel  kiln  then  we  must  provide  heat  and  air 
to  carry  the  heat,  not  only  to  remove  the  moisture  from  the 
ware,  but  to  carry  the  additional  burden  of  combined  water. 
Even  with  air  dried  ware  entering  the  kiln  it  was  found  im- 
practical to  maintain  sufficient  temperature  to  prevent  con- 
densation before  the  combustion  gases  and  hot  air  reached  the 
end  of  the  drying  tunnel.  Otto  Bock,  a  German  engineer,  over- 
came the  condensation  difficulty  by  widening  the  tunnel  at  the 
cold  end,  and  introducing  iron  partitions  between  the  kiln 
walls  and  cars  of  ware.  In  other  words,  he  built  iron  ducts 
in  the  sides  of  the  tunnel  into  which  the  saturated  gases  were 
drawn  and  the  ware  within  the  tunnel  was  heated  by  radiation 
from  the  ducts. 

It  will  readily  be  seen  that  the  development  of  the  car 
tunnel  kiln  has  many  difficulties  to  overcome. 

The  use  of  separate  drying  tunnels  of  any  type  in  which 
to  dry  the  ware  and  to  serve  as  a  storage  room  to  keep  the 
kiln  in  operation  when  the  machines  are  not  in  operation  is 
the  method  in  successful  commercial  use. 

The  heat  from  a  tunnel  kiln,  because  of  the  burden  of  water 
vapor  which  it  carries,  is  not  valuable  for  direct  drying,  but 
in  indirect  drying  we  not  only  realize  the  sensible  heat  in  the 
gases  but  may  also  realize  the  latent  heat  from  any  condensed 
vapor. 

The  combination  of  a  tunnel  kiln  with  a  radiated  heat  dry- 
er of  the  Moeller  and  Pfeifer  type,  or  perhaps  any  type,  will 
result  in  a  maximum  economy  and  this  is  the  present  com- 
mercial development  of  the  car  tunnel  kiln. 

It  is  not  the  province  of  this  discussion  to  consider  receut 
patents,  but  we  will  digress  to  mention  two  which  give  prom- 
ise to  overcome  the  drying  difficulties  of  the  car  tunnel  kiln. 

The  Drayton  kiln  introduces  diaphragms  on  short  cars  at 
regular  intervals,  which  partition  the  tunnel  into  separate 
compartments,  or,  in  other  words,  in  a  measure  convert  the 
tunnel  into  a  chambered  kiln.  The  firing  is  down  draft,  eith- 
er producer  gas,  or  direct  coal  fired,  but  with  this  we  are  not 
here  concerned.  The  movement  of  the  combustion  gases  is 
down  through  the  burning  compartment,  up  through  the  heat- 
ing up  and  watersmoking  compartment,  down  through  the 
dehydration  compartment,  and  thus  up  and  down  until  the 


122  DRYING    CLAY    WARES  . 

gases  are  drawn  off  by  a  fan.  We  do  not  wish  our  readers 
to  accept  the  above  statement  literally.  One  compartment  is 
exclusively  for  the  burning,  but  ahead  of  that  the  compart- 
ments are  not  reserved  exclusively  for  any  particular  stage  of 
the  process — heating  up,  watersmoking  and  drying.  As  in  a 
chambered  kiln  the  gases  are  drawn  ahead  through  several 
compartments  until  the  temperature  and  water  vapor  burden 
have  reached  a  minimum  and  maximum  respectively — in  a 
word,  until  the  dew  point  is  reached — when  they  are  removed 
from  the  kiln. 

A  preheating  flue  extends  from  the  cooling  compartments 
behind  the  fires  to  the  drying  compartments  ahead  of  the  dew 
point  compartment  and  the  initial  drying  is  done  with  air  of 
any  desired  temperature  and  uncontaminated  with  combustion 
gas  and  also  free  from  water  vapor,  except  that  originally  in 
the  entering  air. 

Another  kiln  now  in  the  experimental  stage  covers  each 
car  with  a  muffle  and  when  the  cars  are  connected  in  the  tun- 
nel the  muffles  form  a  series  of  compartment  kilns.  The  fir- 
ing is  done  inside  the  muffles  and  the  air  and  combustion 
gases  pass  from  muffle  to  muffle  as  the  cars  advance  through 
the  tunnel.  The  tunnel  is  a  casing  for  the  moving  compart- 
ment kilns  on  cars,  and  a  drying  tunnel.  Each  muffle-covered 
car  is  loaded  mechanically  inside  the  muffle  with  dried  bricks 
and  on  top  of  the  muffle  is  placed  a  load  of  green  bricks. 
After  the  car  has  passed  the  firing  zone  and  entered  the  cool- 
ing zone,  an  aperture  in  the  top  of  each  or  any  muffle  may  be 
opened,  thus  permitting  the  hot  air  from  the  cooling  bricks 
to  rise  into  the  drying  tunnel  and  by  suction  pass  longitudi- 
nally to  the  end  of  the  tunnel  similar  to  a  progressive  waste 
heat  dryer. 

The  muffles  in  conjunction  form  the  kiln  proper,  in  which 
the  burning  is  done.  The  muffle  roofs  are  the  floor  of  prac- 
tically a  waste  heat  progressive  dryer.  The  movement  of  the 
muffles  carries  the  drying  bricks  forward  and  thus  the  muffle 
roofs  combine  the  dryer  floor  and  brick  carriers.  The  tunnel 
enclosing  the  muffled  cars  has  sufficient  head  room  to  include 
the  drying  bricks  and  is  at  the  same  time  the  drying  and  kiln 
tunnel. 

As  each  car  leaves  the  tunnel  the  burned  bricks  are  re- 
moved from  the  inside  of  the  muffle,  and  the  dried  bricks  on 
top  of  the  muffle  are  placed  inside,  after  which  the  car  is  run 
back  to  the  tunnel  receiving  entrance  and  again  started  on 


DRYING    CLAY    WARES  123 

its  course  through  the  tunnel  after  receiving  a  new  load  of 
green  bricks  on  its  roof. 

The  kiln  is  still  in  the  experimental  stage  but  the  experi- 
ments have  advanced  to  the  extent  of  trying  out  the  burning 
features  commercially,  and,  it  is  reported,  successfully.  The 
drying  features  remain  to  be  proven  but  it  is  believed  that, 
with  the  elimination  of  the  combustion  gases  and  the  water 
vapor  which  the  combustion  gases  carry,  there  will  be  no 
greater  difficulty  in  the  drying  than  there  would  be  in  any 
waste  heat  progressive  dryer.  The  bricks  will  enter  a  warm 
moist  atmosphere  and  as  they  advance  the  temperature  will 
increase,  while  the  degree  of  saturation  will  decrease.  Finally 
they  will  be  subjected  to  the  radiation  and  convection  from  a 
hot  radiating  floor  which  should  remove  the  hygroscopic  water. 

Since  the  above  was  written  a  commercial  kiln,  complete 
in  every  feature,  has  been  put  in  operation  and  the  problems 
connected  with  successful  operation  are  now  being  worked 
out. 


124 


DRYING    CLAY    WARES 


CHAPTER  XII. 
Conservation  of  Heat  in  German  Factories. 

IN  A  PREVIOUS  article  we  described  some  German  meth- 
ods of  constructing  dryers  above  continuous  kilns  to  make 
use  of  the  radiated  heat  from  the  kilns.     This  is  seldom 
done  in  this  country,  partly  because  of  the  great  amount  of 
labor  required  and  partly  because  of  cheaper  fuel. 

The  Germans  build  expensive  plants  where  necessary  to  get 


••: 


the  full  benefit  of  the  fuel  consumed,  while  we  are  very  care- 
less in  this  regard— often  ridiculously  careless. 

Pigs.  58,  59  and  60  show  plan  and  section  of  a  German 
dryer  equipment,  making  extensive  use  of  waste  heat 

The  main  draft  flue  of  a  continuous  kiln  is  connected  with 
the  stack  through  a  series  of  flues,  covered  with  corrugated 
iron  plates,  under  the  dryer  tunnels,  and  the  steam  boiler's 


DRYING    CLAY    WARES 


125 


126 


DRYING    CLAY    WARES 


draft  flue  is  similarly  connected.  Thus  they  get  the  waste 
heat  of  all  combustion,  either  in  the  kiln  or  the  boiler  fur- 
naces, and  all  the  heat  of  cooling  kilns. 

The  exhaust  steam  from  the  engines  is  carried  through  the 
tunnels  in  large  ribbed  pipes  (see  Fig.  60),  which  are  much 
better  radiators  than  the  ordinary  steam  piping  so  largely  used 
in  this  country. 

The  exhaust  from  the  drying  tunnels  is  taken  up  through 
the  roof  of  the  tunnel  into  ducts  leading  to  the  stack,  as  shown 
in  Fig.  59,  and  any  condensation  of  moisture  in  these  ducts 
gives  up  the  latent  heat  of  vapor  to  maintain  the  roof  temper- 


Ft, 


-ss  <5ect lot 

Fij.60 


6J 


ature  and  the  drying  tunnels  are  not  robbed  of  heat  to  supply 
the  radiation  loss. 

The  dryer  is  for  the  pallet  system,  and  periodic  in  opera- 
tion, and  the  projections  in  the  tunnel  walls  are  supports  for 
the  pallets.  The  pallets  are  handled  by  a  lifting  rack  car  or 
truck,  Fig.  61,  holding  the  number  of  pallets  required  in  the 
height  of  the  tunnel. 

We  do  not  present  this  German  dryer  as  one  worthy  of 
adoption  in  this  country,  but  merely  to  illustrate  the  degree  to 
which  the  Germans  carry  conservation  of  heat  and  as  a  hint 
to  clayworkers  of  this  country  that  they  are  allowing  com- 
fortable profits  to  go  to  waste. 


DRYING    CLAY    WARES   .  127 

Moeller  and  Pfeifer  Dryer. 

In  the  development  of  progressive  dryers  the  Germans 
first  advanced  the  principle  that  the  air. should  enter  with 
the  ware  and  that  the  temperature  should  advance  as  moist- 
ure was  taken  up.  It  is  evident  that  the  exhaust  air  leaving 
the  dryer  at  the  hot  end  would  carry  a  large  weight  of  moist- 
ure— pound  for  pound,  or  even  more — and  in  consequence  a 
much  smaller  volume  of  air  would  be  required  than  in  our 
progressive  dryers. 

A  number  of  German  dryers,  using  this  principle,  have 
been  developed,  the  most  interesting  of  which  is  that  of  Moel- 
ler and  Pfeifer.  It  also  illustrates  the  recuperation  of  heat 
values  characteristic  of  German  industrial  operations. 

Figs.  62,  63  and  64  show  this  dryer  in  plan,  longitudinal 
section  and  cross-section. 

In  the  plan,  I  is  the  direct  coal-fired  furnace  at  the  hot  end 
of  the  dryer  G.  The  dryer  is  only  partially  waste  heat,  since 
German  operators  largely  use  continuous  kilns,  in  which  heat 
from  cooling  ware  is  not  available  for  drying. 

The  combustion  gases  from  the  furnace  pass  horizontally 
through  ribbed  radiating  tubes  or  flues  C,  thence  into  flues  H 
connecting  with  stack  or  fan. 

Opposite  the  furnace  is  a  circulating  fan  F,  in  fact,  there 
are  a  number  of  these  circulating  fans,  as  seen  in  the  plan. 

The  fan  F  draws  the  air  through  the  ware,  forces  it  back 
under  the  dryer  floor  through  the  ducts  N,  up  around  the 
radiating  pipes  C,  and  thence  through  the  ware,  a  continuous 
rapid  transverse  circulation  of  the  air  through  the  ware,  with 
no  tendency  to  a  forward  movement  by  the  circulating  fans. 

In  the  plan  and  longitudinal  section  we  have  on  the  dryer 
roof  at  the  cold  end  (X)  of  the  dryer  a  fan  E  connecting  with 
a  cross  duct  G,  and  in  turn  with  down  comer  ducts  D.  The 
ducts  D  connect  with  heating  pipes  A,  which  parallel  the  cars 
of  ware  from  near  the  longitudinal  center  of  the  dryer  to  the 
cold  end.  The  ends  of  these  pipes  at  the  center  of  the  dryer 
enter  vertical  ducts  D,  which  connect  with  a  cross  and  longi- 
tudinal duct  K  to  the  hot  end  over  the  dryer  roof.  The  duct  K 
enters  the  vertical  duct  O  at  the  hot  end  and  this  duct  O  at 
the  bottom  opens  into  the  end  chamber  M  into  which  the  un- 
der ducts  N  discharge.  Thus  the  circuit  with  the  drying 
chamber  is  complete. 

There  are  no  doors  at  the  cold  end  X  and  the  air  here  en- 
ters freely.  It  is  immediately  taken  up  by  the  first  fan  F  and 


128 


DRYING    CLAY    WARES 


DRYING    CLAY    WARES 


129 


the  rotary  circulation  through  the  heating  pipes  and  ware 
started.  The  suction  of  the  fan  E  acting  through  the  hot  end 
of  the  dryer  slowly  advances  the  air  from  fan  P  to  fan  P  un- 
til the  hot  end  is  reached,  and  the  saturated  (or  nearly)  air 
is  drawn  into  the  duct  O  and  started  back  toward  E. 

The  movement  of  the  air  in  the  dryer  is  that  of  a  spiral 
with  a  rapid  lateral  motion  and  a  slow  forward  movement. 
The  air,  as  it  advances  through  the  drying  room,  is  being  used 
to  take  up  moisture  and  thus  dry  the  ware.  When  it  reaches 
the  hot  end  Y  its  usefulness  as  a  drying  medium  is  finished, 
and  upon  its  return  through  the  ducts  and  pipes  its  heat  is 
given  up  to  the  incoming  air. 

Not  only  is  the  sensible  heat  of  the  air  and  vapor  given 


Fig.  64.     Cross  Section. 

up  but  in  cooling  there  must  be  a  large  condensation  of  water 
vapor  from  the  highly  saturated  hot  air  which  returns  to  the 
dryer  the  latent  heat  required  in  the  evaporation. 

The  dryer  losses  are  the  heat  taken  out  by  the  cars  and 
the  ware;  the  heat  in  the  low  temperature  saturated  exhaust 
air  the  volume  of  which  is  very  small;  the  radiation  losses. 
These  losses  are  supplied  by  furnace  I. 

If  there  is  available  exhaust  steam  from  the  factory  en- 
gine it  is  utilized  in  radiation  pipes  B  between  the  furnace  I 
and  the  exhaust  air  heating  pipes  A. 

In  connection  with  a  car  tunnel  kiln  the  furnace  may  be 
dispensed  with.  The  process  in  the  kiln  will  be  extended  to 
the  extent  of  heating  up  and  watersmoking,  and  the  waste 
gases  from  the  kiln  will  have  a  temperature  of  500  degrees  to 
800  degrees  P.  These  gases  will  be  drawn  through  the  radia- 
tors at  the  hot  end  of  the  dryer,  and  will  supply  the  heat  nor- 
mally supplied  by  the  auxiliary  furnace. 


130 


DRYING    CLAY    WARES 


CHAPTER  XIII. 
Progressive  Dryers. 

THE  periodic  tunnel  dryer,  as  its  name  indicates,  is  in- 
termittent in  its  operation.    A  tunnel  is  filled  with  ware, 
then  the  heat  and  air  are  turned  in  and  continued  un- 
til the  ware  is  dry  when  they  are  shut  off  and  the  ware  re- 
moved. 

A  progressive  dryer  is  one  in  which  the  air  volume  is 
constant  and  the  temperature  remains  unchanged  in  so  far 
as  it  is  practical  to  maintain  a  constant  temperture.  In 
the  type  used  in  this  country,  the  heated  air  enters  at 
one  end,  passes  through  the  tunnels  and  escapes  at  the 
other  end.  The  ware,  usually  on  cars,  enters  at  the 
air  exit  end  of  the  tunnels  commonly  termed  the  receiving 
end,  travels  through  the  tunnels  to  the  delivery  end  where 
it  is  removed  from  the  dryer  and  taken  to  the  kilns. 

The  entering  air,  hot  and  with  great  capacity  for  moist- 
ure, first  comes  in  contact  with  the  hot  dry  ware  leaving  the 
dryer.  As  the  air  passes  through  the  tunnels  it  successively 
comes  in  contact  with  cooler,  wetter  ware,  until  at  the  receiv- 
ing end  it  passes  among  the  cold,  damp  ware  just  from  the 
machine.  In  its  passage  through  the  tunnel  it  becomes  cooled 
thereby  lessening  its  capacity  for  moisture  and  at  the  same 
time  it  is  taking  moisture  from  the  ware.  The  dew  point  is 
presumed  to  be  reached  as  the  air  leaves  the  tunnel  at  the 
receiving  end. 

The  drying  condition  is  theoretically  ideal.  The  entering 
hot  air  comes  in  contact  with  ware  in  condition  to  stand  a 
high  temperature  and  which  needs  such  high  temperature  to 
drive  off  the  hygroscopic  water  in  the  pore  spaces  of  the  clay 
mass. 

The  air  leaving  the  dryer  comes  in  contact  with  green 
ware  which  frequently  must  be  carefully  heated  up  without 
any  drying  in  order  to  open  up  the  pore  spaces,  so  that  when 
the  ware  reaches  that  portion  of  the  tunnel  where  drying  be- 
gins the  water  will  be  drawn  to  the  surface  of  the  ware  by 


DRYING    CLAY    WARES 


131 


capillarity  as  rapidly  as  it  is  removed  by  evaporation.  The 
humidity  drying  treatment,  so  necessary  in  safely  drying 
many  clay  wares,  is  automatically  carried  out  in  the  progres- 
sive dryer. 

We  fail  oftentimes  in  the  adjustment  of  the  dryer  and  in 
Its  operation. 

The  normal  condition  of  a  progressive  dryer  is  to  be  full 
of  ware  all  the  time  and  this  is  not  maintained  in  any  plants, 
or  at  least  In  very  few. 

Also  the  escaping  air  should  be  saturated,  or  nearly  so, 
otherwise  there  is  a  loss  of  heat.  If  we  assume  that  a  dryer 
has  been  properly  adjusted  to  a  ware  and  that  the  inlet  tem- 
perature is  200  degrees  F.,  while  the  escaping  air  has  a  tem- 
perature of  100  degrees  F.,  and  the  temperature  midway  of 


100' 


/SO 


Figure  65. 


the  dryer  Is,  let  us  say,  150  degrees  F.,  we  have  the  condition 
illustrated  in  diagram  No.  1,  Fig.  65.  During  the  night  the 
heat  advances  until  in  the  morning  we  have  the  condition  il- 
lustrated in  No.  2  diagram.  After  a  shut  down,  Monday 
morning,  for  instance,  the  heat  has  progressed  until  the  con- 
dition represented  in  diagram  No.  3  is  attained. 

The  setters  In  starting  work  in  the  morning  may  draw 
several  cars  from  each  tunnel,  enough  oftentimes  to  keep 
them  occupied  until  noon.  The  first  bricks  from  the  machine 
come  in  contact  with  a  temperature  and  drying  condition  not 
intended,  and  if  they  stand  it  the  dryer  could  be  changed  to 
more  rapid  drying  conditions  with  fewer  tunnels  in  operation. 
If  the  bricks  do  not  stand  this  more  severe  treatment  the 
fault  is  not  with  the  dryer  but  with  its  operation.  A  progres- 
sive dryer  should  have  receiving  and  delivery  tracks  for  stor- 


DRYING    CLAY    WARES 


age  outside  the  dryer  and  at  intervals  during  the  night  cars 
of  dried  ware  should  be  drawn  and  cars  of  green  ware  en- 
tered. It  is  not  practical  to  store  cars  equal  to  a  day's  out- 
put, but  it  is  common  practice  to  construct  the  storage  tracks 
to  hold  three  cars  on  each  track.  A  tunnel  holds  fourteen  to 
fifteen  cars  and  on  the  basis  of  a  twenty-four  hour  period, 
three  cars  per  tunnel  are  equivalent  to  one-fifth  of  a  day's 
output.  If  the  dryer  has  a  forty-eight  or  seventy-two  hour  pe- 
riod, the  three  cars  are  equivalent  to  two-fifths  or  three-fifths 
of  a  day's  run.  These  storage  cars  aid  materially  in  main- 
taining the  normal  condition  of  the  dryer. 

Operators  often  find  that  the  output  of  the  dryer  is  insuffi- 
cient to  keep  up  the  desired  capacity.  The  first  step  is  usual- 
ly to  increase  the  temperature  which  changes  the  conditions 
to  that  shown  in  diagram  No.  2  or  No.  3,  Fig.  65.  If  the  ware 
cracks,  as  often  is  the  case,  the  difficulty  may,  perhaps,  be 
overcome  by  reducing  the  air  volume  which  retards  the  ad- 
vance of  the  heat  without  necessarily  lowering  the  tempera- 
ture at  the  delivery  end.  With  the  higher  temperature  thus 
maintained  at  the  delivery  end  we  are  enabled  to  drive  off 
the  moisture  remaining  in  the  partially  dried  bricks  without 
damage  to  the  bricks  behind,  since  the  latter  do  not  get  into 
the  high  temperature  zone  until  in  a  condition  to  stand  the 
higher  temperature.  Diagram  No.  4  illustrates  this  condition 
except  that  the  temperature  shading  should  be  deeper  to  show 
a  higher  degree  of  heat. 

There  are  a  number  of  ways  in  which  the  conditions  in 
the  dryer  can  be  changed  without  altering  the  construction 
of  the  dryer,  but  when  these  fail  it  becomes  necessary  to 
make  such  changes  in  the  construction  as  will  give  desired 
results.  These  changes  will  be  considered  under  the  discus- 
sion of  the  several  types  of  progressive  dryers. 


DRYING    CLAY    WARES  133 


CHAPTER  XIV. 
Radiated  Heat  Dryers. 

THE  RADIATED  HEAT  DRYER  is  a  development  from 
the  direct  coal-fired  hot  floor.  The  first  step  in  the 
development  was  to  cover  the  hot  floor  with  tunnels. 
The  next  step  introduced  air  around  the  furnaces  and  into  the 
tunnels,  and  improvements  have  been  made  from  time  to 
time  in  the  air  circulation  and  heat  radiation.  While  the  pame 
"radiated  heat  dryer"  is  appropriate,  yet  the  dryer  really 
makes  use  of  convection  as  well  as  radiation.  The  entering 
air  circulates  around  the  hot  furnace  walls  and  becomes 
heated  by  contact  with  them.  It  then  passes  into  the  tunnels 
and  gives  up  the  heat  to  the  ware  and  to  the  work  of  evapora- 
tion. The  smoke  flues  extend  the  full  length  of  the  tunnel 
under  the  tunnel  floor,  and  the  heat  from  its  walls  is  radiated 
to  the  ware.  The  improvements  in  the  dryer  are  toward  bet- 
ter air  circulation  and  contact  with  the  hot  furnace  walls, 
and  toward  smoke  flue  construction,  which  will  give  a  maxi- 
mum radiating  surface  with  minimum  wall  resistance  to  the 
passage  of  the  heat  by  conduction  from  the  inner  to  the  outer 
surface  of  the  flue  walls. 

The  furnaces  are  at  the  delivery  end  of  the  dryer,  in  a 
pit  below  the  dryer  tracks,  and  usually  and  preferably  are 
built  into  the  dryer,  in  order  that  any  radiation  from  the 
furnaces,  ordinarily  reckoned  as  a  loss,  is  available  in  the 
tunnels  for  drying  purposes.  The  furnaces  are  simple  box 
grates  and  are  coal-fired. 

We  have  worked  out  the  following  table  from  an  article, 
entitled  "A  Contribution  to  the  Technology  of  Drying,"  by 
R.  H.  Minton,  in  Vol.  VT,  Trans.  Am.  Cer.  Soc.  Mr.  Minton's 
calculations  are  based  on  nine-pound  bricks,  containing  in 
round  numbers  two  pounds  of  water  each.  The  air  is  assumed 


134 


DRYING    CLAY    WARES 


p 


DRYING    CLAY    WARES  135 

to  have  an  Initial  temperature  of  60°  F.,  and  to  be  75  per  cent, 
saturated. 

— Fuel  and  Air  Requirement  Per  Thousand  Bricks  Daily — 
No.          Temp.  F.°         Temp.  F.°         Coal  in  Cu.  Ft.  Air 

Hot  End.          Exhaust.          Pounds.  Per  Hour. 

1  82  59  271  246,965 

2  109  68  283  144,309 
176                      86                    281  71,888 

4  314  104  264  38,087 

A  waste  heat  dryer  in  temperatures  and  air  volume  will 
operate  somewhere  between  No.  3  and  No.  4,  inclusive,  while 
a  radiated  heat  dryer  will  fall  below  No.  4.  In  a  waste  heat 
dryer  the  heat  source  is  outside,  and  there  is  no  opportunity 
to  recuperate  the  heat  in  the  dryer.  In  consequence  there  is 
a  greater  fall  in  temperature  from  the  entering  end  to  the 
exit,  and,  other  conditions  being  the  same,  the  volume  of  va- 
por which  can  be  carried  out  is  less. 

In  the  radiated  heat  dryer  the  smoke  flue  (radiating  flue) 
is  constantly  giving  up  heat  to  replace  that  used  in  the  drying 
operation,  and  because  of  this  heat  we  can  remove  more 
moisture  with  a  smaller  volume  of  air.  As  has  been  previ- 
ously stated,  there  is  no  relation  between  air  volume  and 
moisture  volume;  the  air  has  nothing  to  do  with  it,  except,  as 
in  the  case  of  a  waste  heat  dryer,  the  volume  of  heat  is  de- 
termined by  the  volume  of  air. 

Radiated  heat  dryers  can  work  with  a  relatively  small 
volume  of  air.  In  one  instance  we  found  28,000  cubic  feet  of 
air  per  hour  per  thousand  bricks,  but  this  is  greater  than  an 
ordinary  radiated  heat  dryer,  because  of  fan  draft  in  this  par- 
ticular instance,  where  usually  only  natural  draft  is  used. 

The  quantity  of  fuel  per  thousand  bricks  used  in  a  radiated 
heat  dryer  depends  upon  the  volume  of  water  to  be  evapo- 
rated, upon  the  weight  of  clay  and  iron  to  be  heated  up,  upon 
the  dryer  losses,  and  upon  the  efficiency  of  the  operation. 

The  quantities  given  by  Mr.  Minton  are  fairly  representa- 
tive, taking  into  consideration  the  volume  of  water  assumed 
to  be  removed.  Our  records  show  variations  from  200  pounds 
to  380  pounds  per  thousand  bricks,  the  latter  consumption 
being  due  to  improper  construction  and  inefficient  operations. 

Fig.  66  is  a  longitudinal  vertical  section  through  a  tunnel 
of  a  double  track  radiated  dryer,  and  Fig.  67  is  a  plan  view 
below  the  tracks,  and  Figs.  68,  69,  70  and  71  are  vertical 
cross-sections  of  the  same.  The  numerals  marked  on  the  sev- 
eral drawings  indicate  the  following  features  of  the  dryer: 
No.  1  is  the  firing  pit  below  the  track  level;  2  is  the  furnace, 


136 


DRYING    CLAY    WARES 


one  for  each  track  in  the  dryer;  3  is  the  combustion  or  smoke 
flue,  which  extends  the  full  length  of  the  dryer  and  connects 
with  the  main  draft  duct,  4. 

The  dryer  under  description  differs  from  the  usual  radiated 


^Section  a- a     Section  b-b 

Figure  68.  Figure  69. 


Section  c-c   Section  cl-ol 


Figure  70. 


Figure  71. 


heat  dryer  in  that  the  air  and  moisture  at  the  exit  are  drawn 
down  through  the  ware  and  out  through  the  floor  of  the  dryer. 
To  accomplish  this,  double-track  tunnels  are  used,  and  the 
two  smoke  flues  (3)  are  brought  into  one  flue  in  the  center 


DRYING    CLAY    WARES 


137 


of  the  tunnel  near  the  exhaust  end.  The  air  enters  through 
openings  in  the  furnace  fronts — such  openings  being  on  either 
side  and  above  the  furnaces — circulates  around  the  furnaces, 
collects  in  the  hot  air  chambers,  5,  enters  the  tunnel  proper 
through  openings  6,  thence  rises  through  the  ware  and  is 
drawn  the  length  of  the  tunnel.  The  entering  hot  air,  instead 
of  rising  immediately  through  the  ware,  may,  by  means  of 
sheet-iron  plates,  with  graduated  openings  under  the  tracks, 
be  distributed  under  several  cars  as  in  waste  heat  and  other 


± 


I 


Lonjl    'Section 
Figure  72. 


hot-air  duct  dryers.  On  either  side  of  the  single  smoke  flue, 
near  the  exhaust  end,  are  air  and  moisture  ducts  7,  with  per- 
forated floor  (not  shown),  which  connect  with  air  and  moist- 
ure duct  8.  The  smoke  (4)  and  moisture  (8)  main  ducts  lead 
to  stack  or  fan  at  one  side  of  the  dryer,  and  the  air  and  fur- 
nace drafts  are  controlled  by  dampers. 

The  smoke  flues  (3)  are  brick  for  a  distance  of  30  to  40 
feet  from  the  furnaces,  on  account  of  the  intense  heat,  and 
beyond  they  have  brick  walls  covered  with  cast-iron  plates. 


138 


DRYING    CLAY    WARES 


The  idea  is  to  build  these  flues  of  materials  having  the  least 
conductivity  resistance  and  the  largest  possible  radiating  sur- 
face. The  flues  must  be  of  bricks  near  the  furnaces,  but  the 
walls  are  only  4  inches  thick,  and  the  sides,  as  well  as  the 
crowns,  are  exposed  except  buck  walls  at  intervals  to  sup- 
port the  crowns  and  to  carry  the  tracks.  Beyond  the  brick 
flue  cylindrical  iron  pipes  have  been  used  to  the  exhaust  end, 
but  the  more  common  construction  is  that  of  4-inch  brick  side 
walls  covered  with  iron  plates.  Sheet-iron  plates  have  been 


~>r  .in       m  n  r 


Figure  73. 

used  and  sealed  with  sand;  flat,  overlapping  cast-iron  plates 
make  a  simple  covering,  but  greater  radiating  surface  is  had 
from  curved  and  corrugated  plates. 

A  more  usual  form  of  radiated  heat  dryer  exhausts  the  air 
id  moisture  through  the  tunnel  roof  at  the  end.  Figs.  72 
and  73  are  sections  through  the  exhaust  stack  of  such  type 
As  will  be  seen,  the  smoke  flues  (3)  connect  with  an  under- 
ground cross-duct  (4),  which  leads  to  either  side  of  the  dryer 
thence  up  the  back  over  the  dryer  to  a  center  stack.  The 


DRYING    CLAY    WARES 


139 


moisture  outlets  (7)  are  in  the  tunnel  roof  and  the  exhaust 
air  and  moisture  are  directed  into  the  smoke  stack. 

The  hot  radiating  flue  under  the  cars  of  ware  induces  a 
circulation,  as  indicated  in  Fig.  74,  which  we  do  not  get  in 
the  typical  waste  heat  progressive  dryer,  and  with  this  cir- 
culation there  is  slow  progression  of  the  air  toward  the  ex- 
haust end. 

The  value  of  this  circulation  is  too  often  overlooked  In 
the  setting  of  the  ware  on  the  cars.  The  bottoms  of  the  cars 
become  heated  by  direct  radiation,  thus  giving  an  upward  im- 
pulse to  the  returning  air  flowing  under  the  cars.  We  fre- 
quently find  the  ware  set  in  such  a  way  as  to  prevent  the  pas- 
sage of  the  air;  particularly  is  this  true  in  paving  brick  man- 


Figure  74. 


ufacture  where  standard  cars  are  used.  Standard  cars  have 
slats  proportioned  and  spaced  for  standard  bricks,  and  paving 
blocks  on  such  cars  overlap  the  spaces  and  close  them  to  the 
passage  of  the  air. 

A  brief 'discussion  of  the  merits  and  efficiency  of  a  radi- 
ated heat  dryer  may  not  be  out  of  place.  We  are  often  con- 
fronted with  the  inquiry  in  regard  to  the  proper  dryer.  Nat- 
urally, this  cannot  be  answered  without  full  information  in 
each  individual  case,  but  some  general  principles  may  be  dis- 
cussed. 

There  are  many  plants  with  scove,  updraft  or  continuous 
kilns  from  which  little  waste  heat  is  recoverable.  (Note: 
We  may  be  criticised  for  including  the  continuous  kiln  in 


140  DRYING    CLAY    WARES 

this  category,  since  in  a  number  of  installations  heat  for  the 
dryer  is  taken  from  a  continuous  kiln.  We  hold,  however, 
that  it  is  not  waste  heat,  since  it  must  largely  be  replaced 
for  the  kiln  operation,  but  concede  that  such  a  kiln  may  be 
an  economical  place  in  which  to  generate  heat.  However,  as  a 
general  rule,  the  greatest  economy  results  when  the  heat  is 
generated  where  it  is  to  be  used.  The  absurdity  of  the  con- 
tinuous kiln  waste  heat  claim  becomes  apparent  when  one 
realizes  that  in  a  number  of  plants  less  fuel  is  consumed  in 
the  kiln  than  would  be  required  in  the  dryer.  We  cannot  rob 
the  kiln  of  all  its  fuel  value,  and  more,  and  have  left  the  nec- 
essary requirement  for  heating  up,  watersmoking  and  burn- 
ing the  bricks.)  If,  also,  the  steam  power  is  low,  as  is  often 
the  case,  -or  the  plant  is  electrically  equipped,  a  direct  coal- 
fired  radiated  heat  dryer  is  essential.  If  the  steam  power  is 
high,  then  a  steam  pipe  progressive  dryer,  which  is  quite  as 
truly  a  radiated  heat  dryer,  should  receive  consideration. 

A  steam-driven  plant,  with  down-draft  kilns,  is  no  place 
for  a  radiated  heat  dryer,  especially  the  coal-fired  dryer,  but 
if  the  plant  is  electrically  driven  and  near  the  coal  fields, 
where  fuel  is  cheap,  it  may  be  economy  to  lose  the  kiln  waste 
heat  in  order  to  save  the  cost  of  power  required  to  drive  the 
fans  necessary  to  recover  the  waste  heat. 

The  selection  of  the  type  of  dryer,  then,  is  dependent  upon 
the  kind  of  kilns,  the  character  of  the  power,  the  cost  of  fuel, 
and  also  necessarily  upon  the  product  to  be  dried. 

The  radiated  heat  dryer  may  justly  claim  efficiency  when 
in  its  proper  place  and  properly  installed. 

The  heat  carried  away  in  the  combustion  gases  is  essen- 
tial to  create  draft,  both  for  the  furnaces  and  the  tunnels. 

The  relative  high  temperature  at  the  air  exhaust  end 
causes  loss  only  in  case  the  saturation  is  correspondingly  low, 
but  in  view  of  the  small  volume  of  air,  of  its  slow  movement 
through  the  tunnels,  and  of  the  circulatory  tendency,  as  illus- 
trated in  Fig.  74,  there  is  no  reason  why  the  saturation 
should  not  be  practically  complete,  in  which  case  the  high 
exhaust  temperature  becomes  an  efficiency  factor. 

The  circulation  of  the  air  gives  in  some  degree  a  vertical 
movement  through  the  ware  which  is  more  satisfactory  than 
horizontal  draft,  and  the  slow  forward  movement  in  connec- 
tion with  the  rotary  circulation  relieves  us  of  the  necessity  of 
fitting  the  dryer  closely  to  the  mass  of  ware. 

The  horizontal  movement  of  large  volumes  of  air  through 
a  tunnel  dryer  requires  that  there  be  little  free  space  in  or- 
der that  the  air  may  be  forced  among  the  ware,  otherwise 
there  is  little  drying  and  excessive  loss.  It  is  difficult  to 
adapt  such  a  dryer  to  several  kinds  of  ware  on  the  same 
yard— tile  on  double  or  triple-deck  cars,  brick  on  single  or 
double^deck  cars,  pallet  rack  cars— and  in  such  installations 
the  radiated  heat  dryers  are  more  efficient. 


DRYING    CLAY    WARES  141 


CHAPTER  XV. 
Steam  Progressive  Dryers. 

THE  ARRANGEMENT  of  a  steam  progressive  dryer 
differs  from  that  of  a  steam  periodical  dryer  in  that: 
The  steam  piping  is  massed  at  the  delivery  (hot)  end 
of  the  dryer,  the  air  enters  at  the  delivery  end,  moves  hori- 
zontally through  the  dryer,  and,  with  the  vapor,  is  drawn  off 
through  a  suitable  stack  at  the  receiving  (cold)  end  of  the 
dryer. 

The  steam  dryer  may  be  a  series  of  tunnels,  each  equipped 
with  the  proper  amount  of  piping  and  under  individual  control, 
or  more  often  it  is  a  single  large  room. 

The  piping  is  arranged  in  coils,  four  to  six  pipes  deep  at 
the  delivery  end,  two  to  four  pipes  deep  in  the  next  section 
and  two  pipes  in  the  section  most  distant  from  the  delivery 
end.  It  may  extend  the  full  length  of  the  dryer,  or  only  one- 
half  or  two-thirds,  depending  upon  the  character  of  the  clay, 
or  as  the  designer  deems  best. 

Either  exhaust  or  live  steam  is  used  for  heating. 

The  air  volume  required  is  relatively  low,  since  the  heat 
is  not  dependent  upon  the  volume  of  air.  As  the  heat  is  used 
up  by  evaporation  of  the  moisture,  it  is  replaced  by  direct 
radiation  from  the  pipes  under  the  ware  and  by  circulation 
of  the  air  around  the  ware,  among  the  pipes,  and  up  through 
the  ware. 

Fig.  75  is  a  longitudinal  vertical  section  and  Fig.  76  a 
transverse  vertical  section  of  a  steam  pipe  tunnel  progressive 
dryer.  The  air  enters  from  the  outside  at  the  delivery  end 
and  is  distributed  under  the  piping  by  a  floor  with  graduated 
openings.  The  piping  is  under  the  tracks  and  fully  exposed 
within  the  tunnel.  At  the  receiving  end  there  is  no  piping 
shown  and  this  section  of  the  dryer  is  used  for  heating  up 
the  ware  in  a  humid  atmosphere.  If  the  clay  will  stand  se- 
vere treatment  the  piping  may  be  extended  fully  to  the  re- 


142 


DRYING    CLAY    WARES 


DRYING    CLAY    WARES 


143 


ceiving  end  and  drying  begins  very  quickly  after  the  ware 
enters. 

The  drawing  does  not  attempt  to  go  into  any  details  of 
construction  and  is  merely  a  sketch  to  illustrate  the  principle. 

Calculations  of  heat  requirement  and  amount  of  piping 
were  made  for  periodic  steam  pipe  dryers  and  need  not  be  re- 
peated here.  Both  rely  upon  natural  draft  and  use  a  small  air 
volume.  The  draft  in  the  progressive  type  may  be  somewhat 
stronger,  thus  increasing  the  heat  taken  from  the  piping  by 


Figure  76. 

convection,  but  the  difference  is  not  material  and  is  easily 
within  the  conditional  variations  of  the  problem. 

The  steam  pipe  progressive  dryer  for  very  tender  clays 
is  sometimes  built  in  two  compartments  in  series. 

The  first  or  receiving  compartment  is  short  and  without 
any  provision  for  the  admission  or  removal  of  air — in  other 
words,  the  first  compartment,  in  which  a  constant  temperature 
is  maintained,  is  merely  a  heating-up  room.  The  air  in  this 
room  is  necessarily  nearly  saturated  with  moisture  since  no 
air  escapes  except  by  leakage  and  wall  and  roof  absorption. 


144  DRYING    CLAY    WARES 

The  room  also  serves  as  a  storage  room  and  it  is  immaterial 
whether  it  is  kept  full  or  not,  provided  the  ware  remains  in 
the  room  sufficiently  long  to  become  thoroughly  heated  up 
and  put  in  condition  for  the  drying  process.  As  ware  is  re- 
moved from  the  delivery  end  of  the  dryer  proper,  and  the 
cars  of  ware  moved  forward,  the  space  thus  provided  at  the 
receiving  end  is  filled  from  the  heating-up  room.  The  dryer 
proper  is  truly  progressive,  and  since  the  ware  is  previously 
put  in  condition  to  withstand  the  first  stages  in  drying,  the 
piping  may  extend  from  end  to  end. 

A  heating-up  section  is  common  in  other  types  of  dryers, 
but  in  none  of  them  do  we  go  to  the  extent  of  a  separate 
closed  room. 

In  one  or  two  instances  in  the  operation  of  radiated  heat 
dryers  it  was  found  beneficial  to  enclose  the  receiving  tracks 
and  at  the  same  time  remove  the  original  tunnel  doors  at  the 
receiving  end.  The  smoke  and  exhaust  air  and  moisture 
ducts  were  net  extended  to  the  end  of  the  receiving  tracks 
(converted  into  a  receiving  room).  The  ware  on  the  receiving 
tracks  became  heated  in  some  measure  by  circulating  air  from 
the  tunnels,  but  it  was  removed  from  direct  heat  radiation 
from  the  smoke  ducts  and  also  from  the  draft  current  of  air. 
The  ware  in  this  warm  dead  air  space  went  through  a  sweat- 
ing stage  which  has  been  proven  of  value  in  preparing  tender 
drying  materials  for  the  drying  treatment. 

In  the  single  progressive  dryer  this  heating-up  space  re- 
sults from  not  extending  the  piping  to  the  receiving  end,  but 
the  ware  is  subjected  to  the  current  of  nearly  saturated  air, 
although  not  necessarily  so. 

So,  too,  in  progressive  waste  heat  dryers  do  we  provide 
dead  air  heating-up  space  where  the  ware  requires  it.  The 
difficulty  often  is  that  the  need  of  the  ware  is  not  previously 
determined.  A  dryer  is  designed  and  guaranteed  for  rapid 
work  and  afterward  its  adaptation  to  the  ware  requires  ma- 
terial changes. 


DRYING    CLAY    WARES  145 


CHAPTER  XVI. 
Waste  Heat  Progressive  Dryer. 

THE  PROGRESSIVE  waste  heat  dryer  has  found  wide  use 
in  this  country.  As  the  name  implies,  it  is  progressive 
and  uses  only  waste  heat  where  there  is  sufficient  waste 
heat  for  the  work  and  on  this  score  the  margin  is  sufficiently 
small  to  make  suitable  equipment  and  proper  operation  im- 
portant. 

In  round  numbers  under  the  conditions  assumed  in  a  prob- 
lem to  be  discussed  later,  about  1.632,000  heat  units  are  re- 
quired to  dry  1,000  bricks.  If  the  bricks  are  burned  at  a  tem- 
perature of  1850°  F.  (Cone  07)  and  in  cooling  the  heat  is  re- 
coverable down  to  500°  F.,  we  have  an  available  temperature 
of  1850—500=1350°  F.  The  bricks  weigh  6,000  pounds  and 
the  specific  heat  is  .2.  Thus  we  get  from  the  cooling  brick 
1350X6000X12=1,620,000  heat  units.  There  will  be  approxi- 
mately 1,000  brick  in  the  kiln  construction  for  each  1,000 
brick  burned  and  the  average  temperature  of  these  brick  will 
be  about  1000°  F.,  half  of  which,  perhaps,  is  recoverable,  or 
500 X 6000 X. 2=600,000  heat  units.  This  makes  a  total  of 
2,220,000  heat  units,  not  counting  radiation  losses,  which  may 
be  30  per  cent  to  50  per  cent.  If  the  radiation  losses  in  cool- 
ing are  proportional  to  those  in  burning,  we  have  under  the 
above  assumption  insufficient  heat  in  the  cooling  kilns  to  dry 
the  product.  Higher  temperatures  in  burning  will  increase 
the  heat  supply  and  the  balance  may  be  better  or  worse  than 
our  figure,  depending  upon  the  dryness  or  wetness  of  the 
green  bricks.  We  do  not  give  the  above  figures  as  data,  but 
simply  as  an  illustration. 

Besides  the  waste  heat  of  cooling  kilns,  there  is  the  waste 
heat  in  the  exhaust  steam.  If  the  factory  is  using  150  h.  p., 

34.5X150X970X10 

we    have    in    the    exhaust     — =1,003,950  heat 

50 
units  per  thousand  brick. 


146 


DRYING    CLAY    WARES 


34.5  =  Pounds  of  water  per  h.  p.  hour. 
150     =  Total  h.  p. 

970     =Heat  units  per  pound  exhaust  steam. 
10     =  Hours  daily  operation. 
50     =  Capacity  per  day  in  thousands. 

A  waste  heat  dryer  necessarily  involves  the  use  of  a  fan, 
and  the  most  modern  installations  use  two  fans.  In  consider- 
ing the  steam  supply,  we  only  reckoned  ten  hours  operation, 
but  the  fan  operation  will  be  twenty-four  hours  per  day  and 
every  day. 

Probably  30  h.  p.  will  be  required  to  drive  the  dryer  fan 
engines  and  from  these  we  will  recover  34.5X30X970X24= 

34.5X30X970X24 
engines  and  from  these  we  will  recover    - 

50 

481,896  heat  units  per  thousand  brick.  This  is  not  in  addition 
to  the  factory  waste  steam,  but  materially  increases  the  total 
as  previously  estimated  on  a  ten-hour  basis. 

The  losses  between  the  engine  and  the  dryer  is  much  less 
than  those  between  the  kilns  and  the  dryer  and  the  value 
of  the  steam  is  a  material  one  in  reckoning  the  available 
waste  heat  supply.  With  the  exhaust  steam  it  is  evident  that 
there  should  be  sufficient  waste  heat  to  do  the  drying  with  a 
safety  margin  of  100  per  cent,  yet  through  improper  design, 
faulty  construction  and  inefficient  operation,  the  waste  heat 
supply  oftentimes  falls  short  and  has  to  be  supplemented  with  f 
additional  fuel  in  some  way.  Small  and  complicated  hot  air 
ducts  between  the  kilns  and  fan  and  restricted  kiln  connec- 
tions are  frequently  the  cause  of  excessive  loss  in  collecting 
the  heat,  and  failure  to  approximate  the  dew  point  in  the! 
dryer  exhaust  results  in  great  loss  in  the  application  of  thei 
heat.  A  manufacturer  would  not  load  his  cars  with  useles^ 
dead  weight,  but  complacently  moves  a  dead  load  of  useles$ 
and  expensive  air  through  his  dryer.  That  he  may  be  getting 
his  ware  dry  is  no  evidence  that  the  work  is  being  economic- 
ally done. 

The  use  of  the  fans  brings  up  a  factor  of  cost  which  must 
be  considered.  On  the  basis  of  five  pounds  of  coal  per  h.  p. 
hour,  the  fan  engines  will  require  3,600  pounds  of  coal  per 
day,  which,  at.  $2.00  per  ton,  is  $3.60,  or  $0.07%  per  thousand 
brick.  Maintenance  brings  this  cost  above  $0.10.  In  fact,  we 
find  many  fan  installations  where  the  operation  cost  exceeds 
$0.15  per  thousand  brick. 

The  progressive  waste  heat  dryer  is  undoubtedly  the  most 
economical  mechanical  dryer,  but  one  must  not  jump  to  a 
conclusion  that  it  will  be  most  economical  in  every  situation. 


DRYING    CLAY    WARES  147 


We  may  be  burning  a  low  temperature  product  and  not  have 
sufficient  heat  from  the  kiln;  public  service  electric  power 
may  be  available  at  a  less  cost  than  steam;  licensed  engineers 
command  higher  pay.  When  the  waste  heat  supply  is  short, 
we  supply  the  deficiency  by  direct  fired  auxiliary  furnaces. 
This  introduces  fuel  cost  besides  power,  and  often  scumming 
difficulties. 

Difference  in  size  of  kilns  and  in  character  of  product  are 
frequently  annoying  factors — indeed,  they  enter  into  the  cost 
in  proportion  as  they  affect  the  capacity.  When  a  large  kiln 
is  cooling  there  is  an  excess  of  heat,  but  a  small  kiln  may  not 
have  enough  to  carry  the  drying  over  until  a  large  kiln  is 
ready  to  turn  in.  In  changing  from  hollow  ware  to  brick, 
the  hollow  ware  does  not  contain  heat  enough  to  dry  the 
heavier  ware  and  the  operation  of  the  factory  is  delayed  until 
kilns  of  cooling  brick  are  available  for  drying. 

The  use  of  combustion  gases  from  kilns  has  not  been  con- 
sidered and  it  would  materially  change  the  situation.  The 
combustion  waste  gases  and  the  heat  of  cooling  kilns  would 
give  sufficient  heat  without  considering  the  engine  exhaust. 
The  direct  application  of  combustion  gases  rapidly  deterio-  f 
rates  the  dryer  cars,  and  frequently  causes  scumming  and  in 
consequence  the  continued  use  of  combustion  gases  should  be 
through  an  economize^  in  which  the  heat  of  the  gases  serves 
to  heat  air  for  drying  and  in  this  way  we  get  the  benefit  of 
the  heat  without  the  loss  and  damage  by  direct  use  of  the 
gases.  An  economizer  involves  the  use  of  fan  draft  for  the 
kils.  which  in  itself  often  would  be  an  advantage  over  natural 
draft. 

In  the  selection  of  the  dryer  there  are  many  questions  to 
be  considered,  and  one  should  canvass  the  whole  situation 
before  reaching  a  decision. 

Auxiliary  furnaces  to  supply  deficiencies  in  waste  heat 
supply  are  frequently  imperative,  although,  as  previously  men- 
tioned, their  use  is  objectionable.  Where  wood  is  abundant 
its  use  in  the  furnace  removes  the  objections  since  wood  con- 
tains no  sulphur  and  its  products  of  combustion  will  not  in- 
jure the  cars  nor  cause  scumming.  Either  oil  or  natural  gas 
may  be  used,  since  they  are  very  low  in  sulphur,  but  coal  or 
coke  give  a  combustion  gas  seriously  objectionable  in  clay 
ware  dryers. 

Figs.  77  and  78  show  a  wood-burning  furnace  adapted  to 
clay  ware  drying,  or  it  may  be  equipped  with  grate  bars  for 
coal  burning.  The  bridge  wall  is  made  broad  and  filled  with 
brick  checker  work.  This  checker  work  assists  in  maintain- 
ing a  uniform  temperature,  acting  as  a  regenerator  and  at 
the  same  time  brings  the  gases  into  intimate  contact,  thus 


148 


DRYING    CLAY    WARES 


giving  better  combustion.  A  chamber  is  provided  back  of  the 
bridge  wall  to  serve  as  a  dust  collector  and  spark  arrester. 
Figs.  79,  80  and  81  are  plan  and  sections  illustrating  the 
relation  of 'steam  coils,  kiln  ducts  and  auxiliary  furnace  and 
duct.  All  the  air,  whether  from  coils,  kilns  or  auxiliary  fur- 
nace comes  into  a  mixing  chamber  adjacent  to  the  fan,  and 
each  source  of  hot  air  supply  is  controlled  by  damper.  We 
can,  therefore,  use  all  kiln  heat,  all  auxiliary  furnace  heat, 


Figure  79. 


all  steam  coil  heat,  or  any  proportion  of  each.  The  kiln  ducts 
are  usually  placed  underground  and  preferably  so,  but  it  is 
important  that  the  ducts  be  perfectly  underdrained  and  that 
they  be  built  moisture  proof,  as  far  as  possible. 

The  proper  kiln  connection  has  been  a  fruitful  cause  of 
study  and  experiment. 

A  common  method  is  to  connect  the  dryer  duct  with  a 


DRYING    CLAY    WARES 


149 


stack  duct,  each  under  damper  control.  With  the  stack  dam- 
per open  and  the  dryer  damper  closed,  the  kiln  is  under  nat- 
ural draft  and  the  products  of  combustion  pass  into  the  air. 
After  the  kiln  is  burned,  a  change  in  the  dampers  shuts  off 
the  stack  and  turns  the  hot  air  into  the  dryer.  There  are 
three  objections  to  this  method: 

1.     It  is  difficult  to  keep  dampers  tight  and  in  consequence 


<Sectional  Elevation 


Figure  80. 


Fig.  81. 


combustion  gases  find  their  way  into  the  dryer  to  vitiate  the 
air,  scum  the  bricks  and  blacken  them  with  soot,  and  destroy 
the  car  equipment. 

2.  Less  heat  is  obtained  by  down  draft  through  the  kiln 
floor. 

3.  Some  wares  are  damaged  by  forced  draft  among  them 
during  the  cooling  stages. 


150  DRYING    CLAY    WARES 

Regarding  the  dampers  we  have  found  any  single  damper 
unsatisfactory  and  we  have  tried  double  dampers  without  suc- 
cess. Some  yards  close  the  dryer  duct  with  a  brick  wall, 
which  is  replaced  before  each  burn.  After  a  kiln  is  burned 
and  ready  to  connect  with  the  dryer  fan,  the  stack  is  damp- 
ered  off  and  a  hole  is  punched  in  the  dryer  duct  wall.  This 
hole  is  enlarged  as  the  kiln  cools  down,  to  keep  up  the  heat 
supply  by  a  larger  volume  of  air.  Obviously  this  method  has 
no  merit.  It  is  not  tight,  nor  is  it  a  satisfactory  control. 

In  one  instance,  the  dryer  duct  was  below  the  draft  duct 
and  the  two  were  connected  by  a  vertical  flue  from  the  bot- 
tom of  the  draft  duct.  The  top  of  the  vertical  flue  was  re- 
cessed and  in  the  recess  was  bedded  a  heavy  fire  clay  slab, 
thus  closing  the  connection.  The  sulphur  gases  which  leaked 
by  these  dampers  destroyed  the  reinforcement  in  the  dryer 
roof  and  the  roof  fell  in.  This  damper  looked  good  on  paper, 
but  it  failed  in  practice.  A  fairly  tight  sand  sealed  damper 
might  be  constructed  in  this  way,  but  the  deep  dryer  ducts 
would  be  expensive  in  first  cost  and  difficult  to  drain. 

The  simplest  and  at  the  same  time  fully  effective  damper 
is  a  stub  duct  connected  to  the  dryer  duct  by  a  gooseneck. 
When  the  gooseneck  is  removed,  the  connection  between  the 
kiln  and  dryer  is  completely  broken  and  there  can  be  no  com- 
bustion gases  leaking  into  the  dryer. 

Clayworkers  are  familiar  with  the  fact  that  the  bottom 
of  the  kiln  cools  first,  even  though  we  may  be  drawing  air 
down  through  the  ware.  This,  we  think,  is  sufficient  proof 
that  we  cannot  get  all  the  heat  out  of  the  kiln  through  the 
floor. 

One  argument  in  favor  of  the  bottom  draft  is  that  at  any 
time  combustion  gases  can  be  turned  into  the  dryer  to  sup- 
ply any  deficiency.  We  hold  that  there  should  be  no  occa- 
sion to  use  combustion  gases  from  kilns  or  coal-fired  furnaces. 
If  the  steam  equipment  is  proper,  the  deficiency  can  be  made 
up  in  live  steam,  but  if  the  deficiency  is  excessive  and  con- 
tinuous, either  the  kilns  and  dryers  should  be  connected  by 
an  economizer  or  the  waste  heat  dryer  should  be  replaced  by 
some  other  type. 

In  one  plant  the  distributing  ducts  are  the  full  length  of 
the  tunnels,  as  in  Fig.  85.  Suspended  in  these  ducts,  which 
are  very  large,  are  cylindrical  smoke  pipes  connected  with  the 
kiln  draft  flues  at  one  end  and  with  an  exhaust  fan  at  the  other 
end.  All  the  combustion  gases  from  the  kilns  are  drawn 
through  these  pipes  and  the  heat  therefrom  is  available  for 
drying.  The  heat  from  cooling  kilns  is  made  use  of  by  means 


DRYING    CLAY    WARES 


151 


of  a  separate  fan  in  the  usual  waste  heat  progressive  dryer 
manner.  Thus  we  have  an  economizing  system  as  part  of  the 
dryer  construction. 

This  manner  of  using  the  products  of  combustion  would  be 
equally  applicable  to  a  radiated  heat  dryer,  except  both  com- 
bustion gases  and  heat  from  cooling  kilns  would  be  handled 
through  the  smoke  flues  by  the  single  smoke  flue  fan. 

The  most  convenient  place  to  connect  the  dryer  duct  with 
the  kiln  is  through  the  wicket,  and  a  goose  neck  connection  is 
simple  and  effective.  In  the  majority  of  kiln  setting  there  is 
some  space  next  to  the  wicket  and  there  is  always  space  over 
the  wickets  and  in  the  crown.  The  suction  of  the  fan  draws 
the  air  from  around  the  wares  rather  than  through  them,  and 


Fig. 


TT7 


Fig.  83. 


we  not  only  get  more  heat  in  this  way  but  the  wares  are  not 
damaged  by  direct  draft  through  them. 

The  most  efficient  place  to  connect  the  dryer  and  kilns  is 
on  top  of  the  kiln  crown  where  we  fully  recover  the  heat  so 
far  as  recovery  is  possible.  The  wares  under  such  conditions 
cool  entirely  by  radiation.  It  is  not  convenient  to  make  con- 
nections through  the  crown,  and  we  usually  compromise  on  the 
wicket,  or  in  some  instances  a  furnace  connection. 

Figs.  82,  83  and  84  show  a  plan  and  sections  of  a  typical 
waste  heat  progressive  dryer. 

The  hot  air  cross  inlet  duct  from  the  fan  is  tapered  on  bot- 
tom and  one  side  to  correspond  with  the  air  volume  reduction 
as  each  distributing  duct  receives  its  proportion.  The  taper  is 
on  the  distributing  duct  side  so  that  the  end  walls  of  the  dis- 


152 


DRYING    CLAY    WARES 


tributing  ducts  are  in  echelon  and  thereby  each  duct  mechani- 
cally cuts  off  a  proportion  of  air  for  its  supply.  The  distribut- 
ing ducts  have  graduated  openings  under  each  track  for  a  dist- 
ance of  several  car  lengths. 

The  number  of  cars  subjected  to  the  direct  upward  blast  of 
hot  air  through  the  graduated  openings  depends  upon  the  ware. 

If  the  ware  will  stand  severe  treatment  we  extend  the  dis- 
tributing ducts  a  greater  distance,  even  the  full  length  of  the 
dryer,  as  shown  in  Fig.  85,  but  extension  to  this  degree  is  not 
good  practice  because  any  approach  to  complete  saturation  is 


Cross     <J  e  c  f  t  o  rt 
Fig.  84. 


4L__B 


Fig.  85.     Elevation. 


impossible  under  such  conditions,  and  economy  of  operation  is 
coincident  with  complete  saturation. 

A  common  practice  is  to  extend  the  ducts  four  car  lengths 
so  that  one  car  will  be  over  the  inlet  cross  duct  and  three  cars 
over  the  graduated  opening  subjected  to  the  direct  blast  of  hot 
air.  Five  to  eight  car  lengths  subjected  to  the  direct  air  blast 
are  not  uncommon.  Other  things  being  equal,  the  longer  the 
distributing  ducts  and  corresponding  greater  number  of  grad- 
uated openings,  the  more  quickly  the  drying  will  proceed. 

Besides  the  changes  in  the  distributing  ducts  there  are  a 
number  of  modifications  in  the  exhaust  duct. 


DRYING    CLAY    WARES 


153 


I      I 


154  DRYING    CLAY    WARES 

In  one  installation  the  exhaust  ducts  return  under  the 
dryer  floor  to  the  distributing  ducts  and  there  connect  with  an 
exhaust  cross  duct.  We  get  two  advantages  from  this  con- 
struction: It  brings  the  fans  close  together  and  both  can  be 
housed  in  a  single  room  and  driven  from  the  same  shaft.  The 
returning  air  keeps  the  floor  of  the  dryer  warm.  Not  only  the 
sensible  heat  of  the  air  and  vapor  becomes  useful,  but  also  the 
latent  heat  of  any  condensation.  If  the  air  is  saturated  to  the 
dew  point  at  the  exhaust  end  of  the  dryer,  there  must  be  some 
condensation  in  the  return  ducts  to  the  exhaust  fan,  and  the 
heat  from  this  condensation  is  given  back  to  the  dryer  through 
the  dryer  floor. 

-  A  vertical  air  movement  is  much  better  than  a  horizontal 
one,  especially  for  hollow  ware.  In  any  progressive  dryer  we 
have  an  upward  movement  of  the  air  from  the  distributing 
ducts  but  beyond  that  the  movement  is  horizontal  to  the  ex- 
haust and  if  the  ware  is  not  close  to  the  roof  of  the  tunnel 
which  the  hot  air  invariably  tries  to  follow,  there  will  be  poor 
contact  between  the  air  and  ware.  The  exhaust  cross  duct  is 
generally  placed  underground  in  order  to  pull  the  air  down, 
but  it  is  not  very  effective  and  except  at  the  end  of  the  tunnel 
over  the  exhaust  duct  is  without  any  influence  on  the  horizon- 
tal movement  of  the  air. 

To   get   a   vertical   movement,    dryers    are    sometimes   con- 
structed with  exhaust  ducts  having  graduated  openings  simi- 
lar to  the  distributing  ducts,  as  illustrated  in   Fig.   86.     The 
movement  then  is  up  through  the  distributing  ducts'  openings, 
over  an  intervening  space  which  may  be  much  or  little  as  re- 
quired, and  down  through  the  openings  in  the  exhaust  ducts. 
The  exhaust  cross  duct  may  be  at  the  end  of  the  tunnels  or  any 
predetermined  distance  from  the  end  in  order  to  provide  dead 
air  heating  up  space,  and  moreover  the  exhaust  ducts'  open- 
ings nearest  the  exhaust  cross  duct  may  be  closed  to  increase 
the  dead  air  heating  up  space,  but  one  which  will  have  its  floor 
heated  by  the  escaping  air  and  vapor.     Similarly   where  the 
exhaust  cross  duct  is  on  top  as  in  Fig.  85,  it  may  be  placed  at 
any  distance  from  the  end  of  the  tunnel  and  thus  provide  a 
heating  up  space  removed  from  the  direct  air  current. 
Air  Volume  and    Heat   Requirement  for   Waste    Heat   Dryers. 
Assume  for  1,000  bricks  the  following: 
1,000  pounds  metal  in  cars. 
6,000  pounds  clay  in  bricks. 
1,000  pounds  water  in  bricks. 
The  conditions  of  the  problem  are: 
Outside  air  80°  F.,  90%  saturated 
Exhaust  from  dryer  100°  F.,  100%  saturated. 
The   temperature   assumed    represents    average    maximum 


DRYING    CLAY    WARES  155 

summer  conditions,  which  will  require  the  greatest  quantity 
of  air.  A  dryer  equipped  and  adapted  for  these  conditions 
will  have  excess  capacity  at  lower  temperatures  or  less  de- 
gree of  saturation. 

Data  used — specific  heat:' 

Air— .234+.000012  (t-32). 

Vapor— .42+.0001  (t-32). 

Clay— .2. 

Iron— .12. 

Water— 1.00. 

The  problem  is  first  to  find  the  volume  of  air  required, 
and,  second,  the  temperature  to  which  it  must  be  heated  to 
do  the  work. 

From  the  "Vapor  Capacity  of  Air"  table  (page  19),  we  find 
that  air  at  80°  F.  and  90%  saturation  carries  .145  pound  of 
moisture  per  100  cubic  feet,  while  at  100°  F.  and  100%  satu- 
ration the  burden  will  be  .291  pound  of  moisture.  One  hun- 
dred cubic  feet  of  air,  when  raised  from  80°  to  100°  expands 
to  103.7  cubic  feet,  from  the  law  that  the  volume  of  a  gas  is 
directly  proportional  to  its  absolute  temperature 

100  X 

(—  — ,   X=1.037). 

491+80—32      491+100—32 

After  raising  the  temperature  of  the  original  air  to  100°, 
a  volume  of  100  cubic  feet  at  this  temperature  will  contain 

.145 

— =.140  pound  of  moisture,  provided  no  additional  mois- 
1.037 

.140 

ture  is  taken  up,  and  the  vapor  pressure  will  be  =48%  of 

.291 

1.918=.92t).  As  moisture  is  taken  up,  the  volume  increases 
directly  as  the  pressure.  The  pressure  increases  1.918 — .920= 

29 

.998,  and  each  unit  volume  at  100°  becomes =1.036. 

29— .998 

The  volume  increases  to  1.037  because  of  advance  in  tem- 
perature, and  each  unit  volume  of  this  increases  to  1.036  in 
consequence  of  increased  pressure,  due  to  additional  moisture 
taken  up;  therefore,  the  total  volume  of  each  unit  of  original 
air  is  1.037X1.036=1.074.  Since  each  unit  volume  at  100°  and 
complete  saturation  contains  .291  pound  of  water  vapor,  each 
unit  volume  at  80°  and  90%  saturation  when  saturated  at  100° 
will  contain  1.074 X.291=.3125  pound. 

Each  100  cubic  feet  of  incoming  air  under  the  changed 


DRYING    CLAY    WARES 


conditions  has  capacity  for  .3125 — .145=.1675  pound  of  mois- 
ture. 

We  have  1,000  pounds   of  moisture  to   be   removed,   and 

1,000 

therefore  will  require  —    — X  100=597,014  cubic  feet  of  air. 
.1675 

A  common  drying  period  is  twenty-four  hours,  which  deter- 
mines an  air  volume  of  411  cubic  feet  per  minute  per  thou- 
sand bricks. 

The  weight  of  a  cubic  foot  of  dry  air  at  80°  F.  is  deter- 

1.325271XM 

mined  from  the  formula:     -  —  (page  584,  Kent),  in 

459.2+t 

which  M  equals  barometric  pressure  and  t  equals  tempera- 
ture. From  this,  we  find  the  weight  of  a  cubic  foot  of  dry 
air  at  80°  F.  and  29"  barometric  pressure  to  be  .071  pounds. 
Air  expands  as  moisture  is  taken  up,  and  in  one  cubic  foot 
of  the  90%  saturated  air  at  80°  there  will  be  less  than  .071 
pound,  since  the  vapor  is  lighter  than  air  and  displaces  it. 

29 
From  the  formula:  —  we  get  a  value  of 

29— (1.025X.90) 

1.0328  cubic  feet.  This  formula  is  based  on  Boyles  law,  that 
the  volume  is  inversely  proportional  to  the  pressure.  In  the 
formula,  29  equals  barometric  pressure,  1.024  equals  vapor 
pressure  at  80°  F.  (From  vapor  capacity  table.) 

.071 

The  actual  weight  of  air  in  one  cubic  foot  will  be  —   — = 

1.0328 

.06876  pound.  To  this  must  be  added  the  weight  of  vapor 
from  the  vapor  capacity  table,  .00145,  giving  a  total  of  .07021, 
which  is  the  weight  of  one  cubic  foot  of  90%  saturated  air 
at  80°  F. 

We  required  597,014  cubic  feet,  which  is  41,916  pounds 
(597,014 X. 07021)  of  air,  including  the  moisture  naturally  con- 
tained in  it. 

The  problem  now  before  us  is  to  determine  the  tempera- 
ture of  the  air  which  will  be  necessary  in  order  to  bring  suffi- 
cient heat  into  the  dryer  to  do  the  work. 

This  problem  can  only  be  solved  by  a  series  of  approxi- 
mations. 

The  heat  units  required  are  as  follows: 

1.  1,000 X. 12 X temperature  is  the  heat  taken  out  by  cars. 

2.  6,000 X. 2 X temperature   is   the   heat   taken   out   by   the 
bricks. 


DRYING    CLAY    WARES  157 

3.  1,000X20  is  the  heat  absorbed  by  the  water  in  the 
bricks. 

4.  1,000X1,035.6  is  the  latent  heat  of  vapor  at  100°  F. 

5.  41,051(100— 80)  [.234+.0000121 100— 32+80— 32)]  is  heat 
taken  out  by  dry  air. 

6.  866(100— 80) [.42+.0001  (100— 32+80— 32)]  is  heat  taken 
out  by  vapor  originally  in  the  air. 

7.  Dryer  radiation  loss  estimated  at  10%. 

Under  items  1  and  2,  the  temperature  is  indeterminate. 
In  items  3  and  4,  we  assume  that  all  the  evaporation  takes 
place  at  the  exit  at  a  temperature  of  100°  F.,  but  in  reality  it 
is  occurring  at  all  temperatures  between  the  maximum  and 
100°  F.  However,  saturation  is  not  complete  until  the  end  is 
reached.  If  the  evaporation  took  place  at  150°,  the  latent  heat 
would  be  less  and  the  sensible  heat  required  for  the  water 
would  be  greater.  The  resulting  vapor  would  have  a  tem- 
perature of  150°,  but  as  it  approached  the  exit  it  would  cool 
down  and  give  back  to  the  dryer  requirement  its  excess  tem- 
perature. The  heat  taken  from  the  dryer,  therefore,  will  only 
be  that  required  to  heat  the  water  to  the  exit  temperature 
and  to  evaporate  it  at  that  temperature.  This  also  explains 
why  item  4  is  not  made  a  part  of  item  6,  the  latter  item  being 
only  the  moisture  originally  in  the  air,  which  leaves  the  dryer 
at  a  temperature  20°  F.  higher  than  its  initial  temperature. 

In  order  to  solve  the  problem,  we  must  assume  a  tem- 
perature to  which  the  air  must  be  heated,  in  order  to  get 
values  for  the  heat  taken  out  by  the  cars  and  bricks. 

Let  us  assume  a  temperature  of  300°,  an  advance  of  220° 
F.  We  have  then  the  several  items: 

1 26,400  heat  units 

2 264,000  heat  units 

3 20,000  heat  units 

4 1,035,600  heat  units 

5 193,260  heat  units 

6...  7,460  heat  units 


1,546,720  heat  units 
7.  Radiation  loss,  10%     171,858  heat  units 


Tot.  heat  requirement,  1,718,578  heat  units 
The  entering  air  per  degree  advance  in  temperature  re- 
quires: 

41,050  [.234+.000012   (300+80— 64)  ]=9,761 
866  [  .42+     .0001   (300+80—64)]=    391 

10,152 


DRYING    CLAY    WARES 


1,718,578 

—=169°    advance    in   temperature,    or   a   thermometer 
10,152 

temperature  of  169+80=249°  F. 

Evidently  our  assumption  of  300°  F.  was  too  high.  Had  we 
assumed  240°,  the  resultant  determination  would  have  been 
241°,  and  a  third  assumption  of  241°  (1,632,044  heat  units) 
gives  241°,  fractional  degrees  not  considered. 

Suppose  we  have  the  inlet  and  exit  temperatures,  which 
can  easily  be  determined  in  any  dryer,  and  wish  to  determine 
the  volume  in  order  to  adjust  the  fan  to  economic  condition. 

We  will  take  the  temperatures  in  the  previous  problem 
in  order  to  check  results. 

A  cubic  foot  of  the  initial  air  contains  .00145  pound  of 
moisture,  and,  as  previously  determined,  weighs  .07021 

.00145 

pound.     A  pound  of  the  mixture  will  contain  —    — =.02065 

.07021 

pound  of  moisture  and  .97935  pound  of  dry  air. 

The  heat,  in  excess  of  the  initial  heat,  brought  into  the 
dryer  per  pound  of  initial  air  is  as  follows: 

Air  (241—80)  [.234  +  . 000012  (241—32+80—32)]  .97935=37.3823 
Moisture  (241—80)  [.42+. 0001  (241— 32+SO— 32)  ] 

.  .  .02065=  1.4818 


38.8641 
Radiation  loss,  10%= 3.8864 


Available  heat  units  per  pound  of  air= 34.9777 

Each  pound  of  air,  including  the  initial  moisture,  removes 
from  the  dryer: 

Air  (100—80)    [.234+. 000012   (68+48)]   .97935= 4.6106 

Moisture  (100—80)    [.42+.0001   (68+48)]  .02065= .1783 

Total  heat  unit  loss  per  pound  of  air= 4.7889 

Available  heat  for  the  dryer: 

34.9777—4.7889=30.1889  heat  units. 
The  fixed  heat  requirement  is: 

1.  1,OOOX.12X161=    19,320 

2.  6.000X.  2X161=    ..  193200 

3.  1,000X20=   20,'000 

4.  1,000X1,035.6=   1,035,600 

1,268,120  heat  units 

1,268,120 
The  number  of  pounds  of  air  required  will  be:    - 

42,006  pounds  of  air.  3°'1889 


DRYING    CLAY    WARES 


159 


In  our  original  calculation  we  found  41,916  pounds  re- 
quired, a  discrepancy  of  90  pounds,  due  to  the  use  of  whole 
numbers  in  temperatures.  The  difference  is  about  one-half 
of  one  per  cent  and  is  negligible. 

Another  Method  of  Determining  Air  Volume. 

Another,  and  perhaps  simpler,  method  of  determining  the 
air  volume  is  by  use  of  a  formula  developed  by  H.  M.  Prevost 
Murphy  and  published  in  the  Engineering  News  in  1908. 

The  water  vapor  which  a  pound  of  dry  air  can  carry  is 


KH 


found  by  the  formula:     W— - 


-,  in  which  W=pounds  of 


2.036P-H 

water  vapor  per  pound  of  air  at  temperature  t,  and  pressure 
P  in  pounds  per  square  inch.  P  for  29"  barometric  pres- 
sure=29X. 4912=14. 24. 


The  values  of  H 

t|    K    |     H    ||    t 


and  K  are_given  in  the  following  table 


K    1      H 


K    1      H 


.tin:: 

MS9 

78 

r,2"»; 

.958511  156|  .63201    8.744||  234|  .6463]    45.61 

.6115 

.0481 

80 

.6209 

1.024 

ir,.s 

.ma 

9.177|    236   .6467 

47.32 

.6117 

.0526 

82 

.6211 

1.092 

160 

.MM 

9.628  |  238|  .6471 

49.08 

.6120 

.0576 

84 

.6214 

1.165 

162   .6330 

10.10  ||  2401  .6476 

50.89 

.6122 

.0630 

86 

.6217 

1.242 

164|  .6333 

10.59 

242|  .6479 

52.77 

|  .6124 

.MM 

88 

.6219 

1.324 

166   .6336 

11.10 

211    .6484 

64.69 

.6126 

.0754 

90 

.6222 

1.410 

168|  .6340 

11.63 

21.;   .6488 

56.67 

.6128 

.0824 

92 

.6225 

1.501 

170|  .6343 

12.18 

248|  .6492 

58.71 

.6131 

.11!  

94 

.6227 

1.597 

172|  .6346 

12.75 

260|  .6496 

60.81 

.6133 

.0983 

96 

.6230 

1.698 

171    .6350 

13.34 

252|  .6501 

62.97 

.6135 

.1074 

98 

.6233 

1.805 

176|  .6363 

13.96  |    254|  .6505 

65.21 

.6137 

.1172 

100 

.6236   1.918 

178   .6357 

14.60      256   .6510 

67.49 

.6140 

.1279 

102 

.6238!  2.036       1801  .6360 

15.27  II  258   .6514 

69.89 

.6142 

.1396 

104 

.6241 

2.161    1  1821  .6364 

16.97  ||  260   .6618 

72.26 

.6144 

.1523 

mi; 

.6244 

2.294 

184 

.6367 

16.68  M  262|  .6523 

74.75 

.6147 

.1661 

108 

.6247 

2.432 

186 

.6371 

17.43  H  264   .6528 

77.30 

.6149 

.1811 

110 

.6250 

2.578 

188 

.6374 

18.20  ||  266   .6532 

79.93 

.6151 

.1960 

112 

.6253 

2.731 

190 

.6377 

19.00 

268|  .6537 

82.62 

.filfil 

.212" 

114 

.6256 

2.892 

192 

.6381 

19.83 

270|  .6541 

85.39 

.6156 

.2292 

116 

.6258 

3.061 

194 

.6385 

20.69 

2721  .6546 

88.26 

.6158 

.2476 

118 

.6261 

3.239 

196 

.6389 

21.68       274J  .6551 

91.18 

.6161 

.2673 

120 

.6264 

3.425 

198 

>,::!•:< 

22.50  ||  276|  .6555 

94.18 

.6163 

.2883 

122 

.6267 

3.621 

200|  .6396 

23.46  ||  278|  .6560 

97.26 

..;i>;.; 

.3109 

121 

.6270 

3.826 

2"2   .6400 

24.44 

2801  .6565 

100.40 

.6168 

,3880 

126 

.6273 

4.042 

204  |  .6404 

26.47 

282   .6570 

103.70 

.6170 

.3608 

128 

.6276 

4.267 

206J  .6407 

26.53 

284   .6575 

107.00 

.6173 

.3883 

I,  'in 

.6279 

4.603 

208|  .6411 

27.62  | 

286|  .6580 

110.40 

.6175 

.4176 

1!!2 

.6282 

4.750 

21"   .6415 

28.75 

2881  .6584 

113.90 

.6178 

.4490 

134 

.6285 

5.008 

212|  .6419 

29.92      290|  .6590 

117.50 

.6180 

.4824 

i:;t; 

.6288 

5.280 

211    .6423 

31.14    |  292|  .6594 

121.20 

.6183 

.f.lXl. 

138 

.6291 

5.536 

216|  .6426 

32.38  II  294|  .6600 

125.00 

.6185 

.6559 

140 

.6294 

5.859 

218|  .6430 

33.67  ||  296|  .6604 

128.80 

.6188 

.5962 

1  12 

.6298 

6.167 

220 

.6434 

36.01  ||  298|  .6610i 

132.80 

.6190 

.6393 

1  II 

.6301 

6.490 

222 

.6438 

36.38 

300|  .66151 

136.80 

.fiiy.-i 

.6848 

11.; 

.6304 

6.827 

221 

.6442 

37.80 

302|  .66201 

141.00 

.«!% 

.7332 

148 

.CS07 

7.178 

22C 

.6446 

39.27  H  304|  .66251 

145.30 

.6198 

.7846 

150 

.6310 

7.545 

22  S 

.6451 

40.78  ||  306|  .6631J 

149.60 

.6202 

.8391 

If,  2 

.6313 

7.929  |i  230J  .6455 

42.34   M  308   .66361 

154.10 

.6203 

.8969 

154 

.6317 

8.328  ||  232|  .64581  43.95  ||  310   .6641) 

158.70 

160  DRYING    CLAY    WARES 

We  determine  from  the  above  formula  that  a  pound  of  in- 
coming dry  air  at  80°  F.  and  90%  saturation  carries  .02046 

. 6209X1. 024X. 90 

pound  of  water  vapor    ( —  — =.02046). 

2.036X14.24—1.024 

The  weight  of  a  pound  of  air  with  water  vapor  will  be 

.02046 
1  02046  pounds.    A  pound  of  the  mixture  will  have  — 

1.02046 
.02  pound  of  water  vapor  and  .98  pound  of  air. 

This  weight  of  dry  air  at  100°  F.  and  100%  saturation  will 

.6236X1.918X.98 
carry  -  -=.0432  pound  of  water  vapor. 

2.036X14.24—1.918 

Bach  pound  of  the  incoming  air  mixture  has  capacity  to 

remove  from  the  dryer   .0432— .02=.0232  pound  of  moisture. 

Since  there  are  1,000  pounds  of  moisture  to  be  removed 

1,000 
per   thousand   bricks,   there   will   be   required   -     — =43,100 

.0232 
pounds  of  initial  air  mixture. 

This  method  of  figuring  gives  us  a  result  of  2.8%  higher 
than  the  method  previously  used. 

The  weight  of  a  cubic  foot  of  dry  air  at  80°  F.  is  deter- 

1.325271  M 
mined  from  the  formula  —  -    (page   584,   Kent),  in 

459. 2+t 

which  M   is   barometric  pressure  in  inches   of  mercury  and 
t  equals  temperature. 

From  this  we  determine  the  weight  of  a  cubic  foot  of  dry 
air  at  80°  F.  and  29"  barometric  pressure  to  be  .07128  pound. 

Air  expands  as  moisture  is  taken  up,  and  in  one  cubic  foot 
of  90%  saturated  air  at  80°  F.  there  will  be  less  than  .071 
pound,  since  vapor  is  lighter  than  air. 

29 
From  the  formula  —  —  we  determine  that  one 

29— (1.024+.90) 

cubic  foot  of  dry  air,  in  taking  up  moisture  to  the  degree  of 
90%  saturation,  expands  to  1.0328  cubic  feet.  This  formula 
is  based  on  Boyles  law,  that  the  volume  is  inversely  propor- 
tional to  the  pressure.  In  the  formula,  29=barometric  pres- 
sure, 1.024=elastic  force  of  vapor  at  80°.  (H  in  above  table.) 
The  actual  weight  of  air  in  one  cubic  foot  of  the  mixture 

.07128 

will  be  —  — =.069  pound 
1.0328 


DRYING    CLAY    WARES  161 

A  pound  of  mixed  air  and  vapor,  as  previously  determined, 

•  •mains  .98  pound  dry  air  and  .02  pound  water  vapor.     The 

weight  of  water  vapor  in  a  cubic  foot  of  air  at  80°  and  90% 

saturation  is  found  from  the  proportion: 

X=.00141.  By  Seger's  formula,  used  in  our  capacity  table, 
this  value  is  .00145. 

The  weight  of  a  cubic  foot  of  the  air  mixture  is  .069+ 
.00141=.07041  pound. 

43,100 

The  volume  of  air  required  will  be  —    — =r612,129  cubic 

.07041 

feet  per  thousand  bricks.  Under  the  other  method  of  de- 
termination, the  volume  was  597,014,  a  difference  of  about 
10  cubic  feet  of  air  per  minute  per  thousand  bricks. 

Either  method  gives  results  sufficiently  accurate  for  any 
practical  purpose. 

Calculations  along  this  line  should  be  of  great  value  in 
adjusting  a  waste  heat  dryer  to  the  highest  efficiency.  We 
can  determine  the  temperature  by  thermometers,  the  degree 
of  saturation  by  wet  and  dry  bulb  thermometers  or  the  more 
convenient  diagramatic  modifications  of  the  same,  and  the 
air  volume  by  anemometers  or  Pitot  tubes.  With  such  data 
we  should  be  able  to  properly  adjust  the  operation  of  the 
dryer. 

In  the  waste  heat  dryer,  the  highest  efficiency  will  come 
from  an  initial  high  temperature.  The  advantage  comes  in 
several  ways: 

1.  The  high  temperature  means  materially  less  volume. 

2.  Less  volume  means  slower  progress  through  the  tun- 
nels,  with   consequent   proportionately   greater   reduction   in 
temperatures. 

3.  Less  volume  and  lower  exit  temperature  assure  more 
complete  saturation. 

4.  Less    volume    takes    correspondingly    less    heat    out 
through  the  exhaust. 

5.  More  complete  saturation  means  less  trying  conditions 
on  the  ware  entering  the  dryer. 

6.  Less  volume  means  less  power  to  drive  the  fans. 

The  size  of  the  fan  is  always  a  perplexing  question,  and 
one  that  usually  has  to  be  decided  before  the  exact  data  in 
regard  to  moisture  and  perhaps  temperature  can  be  deter- 
mined. Fortunately,  a  fan  has  a  wide  range;  and,  provided, 
we  install  one  of  sufficient  size,  it  can  be  adjusted  to  any  de- 
sired volume. 

The  capacities  of  fans,  as  given  by  the  manufacturers  of 


162  DRYING    CLAY    WARES 

such  equipment,  do  not  apply  to  our  conditions,  nor  would 
any  capacity  table  be  of  general  application.  We  draw  the 
air  through  a  simple  to  complex  checker  work  and  flue  sys- 
tem, and  force  it  into  and  through  the  dryer  against  a  re- 
sistance much  greater  than  an  ordinary  heating  system. 

It  has  been  our  practice  to  determine  the  actual  air  re- 
quirement under  adverse  conditions,  and  then  select  a  fan  of 
double  this  capacity  at  three-fourths  ounce  pressure. 

The  piping  required  for  a  waste  steam  heat  application 
will  depend  upon  the  weight  of  exhaust  steam. 

Under  the  discussion  of  periodic  dryers,  we  presented  a 

T-t 

formula   (R= )   to  determine  temperatures  possible  from 

k 

coil  heaters.  R=rise  in  temperature,  T— temperature  of  the 
steam,  t=temperature  of  the  air,  k=factor  from  table  accom- 
panying the  table.  If  the  steam  pressure  is  4.3  pounds,  which 
would  approximate  5  pounds  back  pressure  on  the  engine,  its 
temperature  will  be  225°.  The  temperature  obtainable  from 
a""six-section  heater  with  air  velocity  of  900  feet  per  minute, 

225—80 

air  temperature  of  80°  will  be  T=(R+80) — 1-80=177° 

1.49 

F.  We  ordinarily  install  eight  sections,  but  the  last  two  are 
arranged  for  high  pressure  steam,  to  enable  us  to  supple- 
ment with  live  steam  when  there  is  a  shortage  of  kiln  waste 
heat. 

Each  pound  of  steam  condensed  at  atmospheric  pressure 
delivers  970.4  heat  units. 

Each  pound  of  air  requires: 

Air  (177—80)  [.234+.000012  (193)]  .97935=. .  .22.45  heat  units 
Moisture  (177—80)  [.42+.0001  (193)]  .20265=  .88  heat  unit 

23.23  heat  units 

970.4 

Each   pound    of   steam,    therefore,   will   heat  =41.6 

23.23 
pounds  of  air. 

If  there  are  100  horsepower  available,  we  will  have  100 X 
34.5=3,450  pounds  of  steam  per  hour,  and  this  will  heat 
3,450X41.61=143,520  pounds  of  air  per  hour,  or  2,392  pounds 
per  minute. 

In  the  discussion  of  periodic  dryers,  we  determined  2.79 
square  feet  of  heating  surface  in  each  row  of  pipes  per  square 
foot  of  free  area.  This  gives  2.79X24=66.96  square  feet  in 


DRYING    CLAY    WARES  163 


six  sections,  and  this  radiating  surface  heats  900  cubic  feet, 

2,392 

or    .07021X900=63.19    pounds    of   air    per   minute.     X 

63.19 

66.96=2,534  square  feet  of  radiating  surface,  or  7,602  lineal 
feet  of  one-inch  pipe  to  condense  the  steam  from  100  horse- 
power. 

This  result  is  only  approximately  correct,  since  it  does  not 
take  into  consideration  any  radiation  loss  from  the  coils,  and 
in  consequence  some  of  the  steam  will  be  required  to  main- 
tain this  loss;  but,  as  this  would  reduce  the  amount  of  piping 
required,  the  result  obtained  gives  us  a  desired  factor  of 
safety,  and  no  correction  should  be  made. 

The  addition  of  two  sections  using  live  steam  will  decrease 
the  radiating  surface  required. 

T-t 

From  the  formula  =R,  we  determine  that  live  steam 

k 

at  60  pounds  in  two  sections  of  heater  coils  will  advance  the 
temperature  from  177°  to  218°.  This  advance  of  temperature 
will  require  a  heat  consumption  of  9.937  heat  units  per  pound 
of  air. 

We  have  previously  determined  that  a  pound  of  air  from 
80°  to  177°  requires  23.33  heat  units,  making  a  total  of  33.267 
heat  units  to  heat  a  pound  of  air  from  80°  to  218°. 

We   found   that  a   pound   of   exhaust   steam   heats   41.61 

904 

pounds  of  air  to  177°,  and  similarly  determine  ( )  that 

9.937 

a  pound  of  live  steam  will  heat  90.96  pounds  of  air  from  177° 
to  218°. 

The  relative  steam  consumption  is  proportional  to  the 
weight  of  air  heated,  and  may  be  determined  from  the  equa- 

1          1— x  1          1— x' 

tions  = and  —    — = ,  in  which  x  equals 

132.57       41.61  132.57       90.96 

.686  for  exhaust  steam  and  x'  equals  .314  for  live  steam. 

The  live  steam  radiating  surface  per  square  foot  of  free 
area  is  2.79X8=22.32  square  feet. 

One  hundred  horsepower  in  live  steam  will  heat  3,450  X 
90.96=313,812  pounds  of  afr  per  hour,  or  5,262  pounds  per 

5,262 
minute.  -  — X  22.32=1,859  square  feet  of  radiating  surface 

63.19 
to  condense  100  equivalent  horsepower  in  live  steam. 


164  DRYING    CLAY    WARES 

We  found  that  2,534  square  feet  would  be  required  for  ex- 
haust steam. 

If  both  are  used  at  the  same  time,  the  surface  of  each 
will  be: 

1,859 X. 314=    584  square  feet  of  live  steam  pipe  surface. 

2, 534 X. 686=1,738  square  feet  of  exhaust  steam  pipe  surface. 


2,322  total  surface  required,  or  6,966  lineal  feet 
of  piping. 

Making  due  allowance  for  uneconomical  dryer  operation, 
this  amount  of  piping  will  suffice  for  30,000  bricks  per  day, 
each  brick  containing  one  pound  of  water. 

It  will  be  noted  that  the  result  bears  no  relation  to  the 
dryer  capacity,  being  simply  dependent  upon  its  volume  of 
steam  available. 

If  we  wished  to  determine  the  piping  required  to  supply 
heat  to  dry  1,000  bricks  under  the  conditions  of  the  original 
problem,  we  must  first  determine  the  temperature  obtainable 
from  exhaust  steam,  which  in  the  above  problem  we  found  to 
be  177°.  We  will  assume  that  two  sections  are  to  be  used  for 
live  steam.  In  the  formula 

T-t 

R= and  the  table  of  the  factor  k  included  in  the  dis- 

k 

cussion  of  periodical  dryers,  we  know,  or  can  easily  deter- 
mine, the  value  of  R=64,  t=177,  k=3.13;  and  from  these,  by 

T— 177 

substitution  in  the  formula  (64= —      — ),  we  determine  that 

3.13 

the  live  steam  must  ha\e  a  temperature  of  377°,  which  is  a 
boiler  pressure  of  175  pounds. 

This  steam  temperature  is  higher  than  we  would  have  in 
practical  operation,  but  it  can  be  reduced  by  the  use  of  a" 
greater  number  of  sections  of  live  steam  coils. 

The  number  of  sections  required  is  easily  determined. 
The  temperature  of  the  air  entering  the  live  steam  sections 
is  177°,  and  it  must  be  advanced  to  241°,  an  increase  of  64°. 

T-t  T— 177 

Substituting  in  the  formula  R= ,  we  have  64= — 

k  2.30 

T— 177 

for  three  sections  and  64= for  four  sections.    The  first 

1.91 

equation  gives  324°  for  T,  which  corresponds  to  a  steam  pres- 
sure of  approximately  80  pounds,  and  the  second  equation 


DRYING    CLAY    WARES  165 

gives  299°,  which  is  slightly  in  excess  of  50  pounds  pressure. 
Either  of  these  pressures  are  commonly  used  in  brick  plants, 
and  for  our  problem  we  will  use  four  sections  of  live  steam 
piping. 

Since  the  air  velocity  through  the  heater  is  assumed  to  be 
900  feet  per  minute,  each  square  foot  of  free  area  passes 
900 X. 07021=63.2  pounds  of  air  per  minute. 

As  already  shown,  the  exhaust  steam  in  six  sections  heats 
the  air  to  177°  F,  and  there  must  be  heat  developed  in  the  live 
steam  sections  to  advance  the  temperature  to  241°  F. 

The  total  heat  development  will  be: 

1.  63.2   (177—80)    [.234+.000012   (193)]   .97935=1,418.2 

63.2  (177—80)   [.42+.0001  (193)]  .02065=....      55.6—1,473.8 

2.  63.2  (241—177)  [.234+.000012  (354)]  .97935=    943.8 

63.2  (241—177)   [.42+.0001  (354)]  .02065=...      38.0—    981.8 


Total  heat  units  per  sq.  ft.  of  free  area= 2,455.6 

The  total  heat  units  required  for  1,000  bricks  is  1,632,044, 
which,  on  the  basis  of  a  twenty-four-hour  drying  period, 

1,632,044 

would  be  —       — =1,133.4  heat  units  per  minute. 
1,440 

1,133.4 
The  free  area  per  thousand  bricks  will  be:  — =.46 

2,455.6 
square  foot. 

The  piping  included  within  this  area  will  be  2.79X6X4X 
.46=30.80  square  feet  for  exhaust  steam  and  2.79X4X4X.46= 
20.53  square  feet  for  live  steam,  making  a  total  of  51.33  square 
feet,  or  154  lineal  feet  of  one-inch  pipe  for  1,000  bricks. 

X,,te — Since  tlie  weight  of  air  required  for  1,000  bricks  is  a 
factor  in  determining  the  total  heat  requirement,  we  can  deter- 

41,916 
mine  the  free  area  required  direct  from  the  weight  of  air,  = 

29.1 

29.1   pounds  of  air  per  minute,   and  =.46   square  foot   of   free 

,  63.2 

area. 

The  amount  of  piping,  as  above  determined,  is  that  re- 
quired for  perfect  operation  of  the  dryers;  but  dryers  are 
seldom  operated  economically,  and  to  whatever  extent  they 
vary  from  the  theoretical  operation,  in  the  same  proportion 
will  the  heat  requirement  increase,  and  correspondingly  must 
the  steam  heating  system  be  enlarged. 

In  many  installations,  double  the  theoretical  amount  of 
piping  is  installed,  which  is  an  admission  that  practical  opera- 
tions may  be  only  50%  perfect;  but  in  no  instance  need  the 


166  DRYING    CLAY    WARES 

piping  be  more  than  50%  in  excess  of  the  theoretical  require- 
ment. 

However,  excess  piping  does  not  necessarily  involve  loss 
of  heat,  because  the  steam  is  condensed  only  in  proportion  as 
the  heat  is  removed  from  the  piping  by  the  air.  The  greatest 
loss  in  waste  heat  dryers  is  in  using  an  excess  of  air,  which 
must  be  heated  up. 

About  12%  of  the  total  dryer  heat  requirement  is  used  in 
heating  the  air,  and  if  the  air  is  only  50%  saturated,  as  it 
leaves  the  dryer  the  requirement  for  the  air  becomes  24%. 

The  boiler  horsepower  required  per  thousand  bricks  can 
easily  be  approximately  determined,  since  we  only  need  to 
divide  the  heat  requirement  per  hour  by  the  heat  value  of  a 

1,632,044 

pound  of  steam,  —  — =70   pounds   of  steam,   or  about 

24X970.4 

2  H.  P.    A  safety  margin  of  50%  would  increase  this  to  3  H.  P. 
Finis. 


Economies  in  Brickyard 
Construction  and  Operation 

\]O  MATTER  how  successful  you 
have  been  in  your  business,  no 
matter  how  economically  you  are 
operating  your  plant,  no  matter  how 
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getting  from  your  present  methods, 
the  book  will  prove  of  inestimable 
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WHY?  Because  it  is  practical,  cov- 
ers the  field  thoroughly,  points 
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you  average  cost  and  shows  you 
what  the  cost  of  the  manufacture 
of  your  brick  should  be. 

It  is  the  best  offer  for  the  price  we 
ever  made. 

Price    -    -    $1.00  . 

T.  A.  Randall  &  Co. 

Indianapolis,  Ind. 


CLAYS;     THEIR    OCCURRENCE,    PROPERTIES    AND 

USES    Henrich   Ries     $5.00 

THE  EFFECT  OF  HEAT  UPON  CLAYS 

A.  V.  Bleininger       2.00 

ECONOMIES  OF  BRICKYARD  CONSTRUCTION   AND 

OPERATION    Ellis   Lovejoy        1.00 

SCUMMING    AND    EFFLORESCENCE 

Ellis   Lovejoy         .50 

TABLES  OF  ANALYSES  OF  CLAYS 

Alfred    Crossley       2.00 

THE   CLAYWORKERS'    HAND    BOOK 2.00 

HOLLOW    TILE     HOUSES 

Frederick   Squires       2.50 

MODERN    BRICKMAKING     

Alfred    B.   Searle       5.00 

CLAY   GLAZES   AND    ENAMELS 

Henry   R.  Griffen        5.00 

VITRIFIED    PAVING    BRICK 

H.  A.  Wheeler       2.00 

ENGINEERING    FOR    LAND    DRAINAGE 

C.  G.   Elliott       2.00 

TILE    UNDERDRAINAGE    

J.  J.  W.  Billingsley         .25 

RADFORD'S  BRICK  HOUSES  AND  HOW  TO  BUILD 

THEM    ••  William  A.   Radford        1.00 

Any  or  all  of  these  books  will  be  mailed,  postage 
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UNIVERSITY  OF  CALIFORNIA  LIBRARY 

Los  Angeles 
This  book  is  DUE  on  the  last  date  stamped  below. 


Ata  6 


NOV  2  8  195V 


(0CT  1  9  1955 
$07  12 '362 

TCT  10 

'JAft  i  o  mi 


5  1976 


NOV     9 

BfclNJH 


SCIEUCES  LIBRAF 


«B1 


MAY  29  1981 
STACK 

JAW  121987 
ANNEX 

-M, 


fi:r 


ot 


ANNEX 


TP 
81 


MDQUARV 

STACK 


